I don't really know where to start, so lets just dive right in:
Clear & Sprint - Bandwidth & an Open Relationship:
Even if LTE does end up winning in the global market, Sprint & Clear (from here on out, if I mention one, assume the other is mentioned too) can easily switch. Their 2.5Ghz spectrum is widely used worldwide for both LTE & WiMax, so not only can they easily switch in software, they will have better global roaming potential than the other big carriers here in the USA that are using 700Mhz spectrum for there nets.
Clear is running tests this year in both TD-LTE and FDD-LTE. They are clearly shaping up to be one hell of a 4G provider, and even a backbone provider with their large WiMax buildout so far done. (WiMax is a nearly perfect technology for wireless backhaul, in case you didn't know)
Also, Sprint has MASSIVE spectrum holdings in the 2.5Ghz channels, so that gives them many more advantages that I'll get into later.
Frequency, frequency, frequency!
I cannot stress how important this is! Everyone is saying how much better LTE is than WiMax because of its better building penetration & lower build out costs. Being on 700Mhz here in the USA, it will require ~1/4th of the equipment vs WiMax to get the same coverage area & building penetration.
Thats fine & dandy, until you talk global roaming. See, in the EU, LTE is actually slated for the 2.6Ghz channel, and WiMax is still on 2.5Ghz. So here in the USA, LTE probably does have an advantage coverage & cost wise to carriers, but it also hamstrings them in multiple ways. In Europe, with it on the 2.6 channel it will actually be on par if not slightly worse than WiMax coverage & penetration wise. It will also cost about the same to roll out.
Obviously being on 700Mhz here in the USA, people will be able to roam between Verizon & AT&T, and anyone else on that channel. The problems come when you go overseas. As mentioned above, in Europe LTE is on a completely different freq than here, so global roaming without multi-band radios is pretty much out. This increases cost for devices.
Clear will be using 2.5Ghz spectrum for LTE (should they switch) so they should be able to roam globally, although they may not. This is a HUGE advantage.
Size DOES matter!
No, you pervs, not like that. :P
Because of the vast spectrum that Sprint holds, they can take advantage of it and provide much higher throughput over the same technology. While VZW & AT&T are limited spectrum wise because of using 700, Sprint isn't. Most LTE carriers in the US can only offer 10Mhz channels for upload & download. This leads to the weak (relatively) speeds of 5-12Mbps down Verizon is promising at launch.
On the other hand, Clear can take advantage of all that spectrum & offer channels of
Is It True 4G?
It depends. LTE is a true 4G standard no matter how you slice it.
WiMax-16e is what Clear currently has rolled out. It is NOT true 4G. It has most of the qualifications (full IP backbone network, etc) but it doesn't meet speed requirements. Fixed you have to (theoretically) be able to provide 1Gbps downlink & 100Mbps mobile to qualify as 4G. LTE (if you use the right frequencies & have the spectrum to provide wide enough channels) can do that. 16e can't. Enough said.
BUT! WiMax-16m (WiMax2, as it's been branded) is a true 4G standard. It was finalized as a standard this summer, and equipment providers (Alcatel-Lucent, Motorola, etc) are expected to be able to provide backend gear for it by early next year & user devices by the end of next year. This is where WiMax really can compete with LTE. Think of the WiMax to WiMax2 upgrade as the HSPA to HSPA+ rollouts going on on T-Mobile & Bell/Rogers in Canada. Another plus for WiMax 2 (16m) is that it is completely backward compatible with WiMax (16e), again just like HSPA devices are compatible with HSPA+ networks.
LTE does offer a standardized voice transmission method, whereas WiMax (2) doesn't. This is a big thing for carriers, and I'm not going to say it doesn't matter, because it does. Sprint & any other WiMax provider worldwide will have to maintain their GSM or CDMA2000 networks to keep providing voice. That also means that mobile devices will have to provide dual-mode CDMA/WiMax or GSM/WiMax chips to stay connected. This could change. They could implement it down the road, but in its current state, WiMax can't do it. OTOH, EvDo devices also have to be dual mode, since the EvDo standard that CDMA carriers chose couldn't do voice, whereas different techs (such as EvDv from Qualcomm) could do both. So it's not really new to many of them, just something to consider.
Wrapup (my opinion):
Both are great technologies if done right. Right now, WiMax has the advantage because its more rolled out & it has the spectrum available to operators to provide higher speeds. That could easily change in the future, especially if more telcos can provide larger channels for LTE to reach its full speeds. WiMax 2 could also be a game changer in the industry.
In all actuality, the technologies are almost identical. It isn't like HD-DVD vs BluRay, where there was a clearcut winner. There won't be that here, both are excellent technologies & will continue to coexist. Much like Cable vs. DSL in the landline world. WiMax offers great technology as wireless backhaul & last mile delivery for rural broadband, but is also becoming a good access technology for 4G wireless. LTE was designed by carriers & for carriers as the natural progression to HSPA to carry voice as well as data. It is an access network at heart. WiMax is more flexible, it can do access or backhaul. It really comes down to carrier choice & what freqs they have available to them.
Hope this helps clear some stuff up. I've seen a lot of uneducated posts about this & it really annoys me.
EDIT - More info:
Topography is another huge factor. For some markets LTE will be a better choice, and for others WiMax will. Again it comes down to what freqs the telcos have available to deploy on.
Also, I forgot to clarify a couple more things about Clear's LTE trials. I mentioned that they were using TD-LTE & FDD-LTE (a good article on them here). A key difference is that TD is able to be used on the same unpaired freqs as WiMax is, so where carriers that only had access to the unpaired freqs before had to go with WiMax can now go with a variation of LTE (although still not the same as the normal carriers). FDD is what most telcos already have access to today, so they are building on it. Clear has both. Another win. Clear's TD-LTE trials are using paired 10Mhz channels for a total of 20Mhz, which will provide speeds potentially faster than WiMax but slower than FDD-LTE. Their FDD-LTE trials will be using paired 20Mhz channels for 40Mhz total.
Several more good articles on the subject here, here, and here.
WiMax is currently seen as the predominant tech of choice in India because of the abundance of unpaired spectrum available there. Good articles on that here & here. Intel is a huge backer of WiMax in India as well as the US, and are offering SoC's & laptop chipsets with WiMax integrated. They are also offering standalone Mini-PCIe WiMax cards. See more info on big WiMax players in the WiMax Forum group.
I'm trying to be as unbiased as possible here & give the (dis)advantages of both techs here. There are links in defense of both techs above, and I'm not a "fanboy" for either one.