My History with the Sensation: I've had the Sensation 4G (T-Mobile US) since February 2012. At the time, it was a good phone and among the first crop of US smartphones to get legit ICS. I've used all different ROMs and kernels in my stint with the Senny. Everything from 4.0 to 4.3, Sense, De-Sensed, AOKP, CM, PA, etc.
Why I picked the G2: Any phone I purchase, I expect to keep for 2 years. This means that it must have top-tier specs to remain relevant for so long. The G2 is one of the few phones available in the US with a Snapdragon 800. It has a very small bezel and has all the features you'd expect in a flagship device: 1080p screen and video recording, 2GB RAM, NFC, IR, etc. I chose it over the HTC One and Galaxy S4 specifically because it has a better SoC, better battery, and is just barely bigger (such a small difference you would hardly notice it). Also, despite having a MUCH bigger battery than the HTC One, it manages to be thinner.
First Impressions of the G2: The first thing you will notice going from Senny to G2 is the screen. It is almost a full inch bigger (4.3 vs 5.2), the resolution is full HD, and the pixel density is incredible-surpassing even the iPhone's "Retina Display". Colors are brighter and richer. The screen is just absolutely gorgeous.
The second thing that strikes you is that despite the screen being so much bigger, the phone itself isn't too much of a behemoth compared to the Senny. It is only 1/2 inch (12mm) taller and 1/4 inch (6mm) wider. It's also slightly (2mm) thinner than the Senny, and weighs 5g less. It will certainly take some getting used to, but I expect to be a pro in no time flat.
*Snapdragon 800: Owning the Sensation, you become accustomed to waiting. Opening even simple apps, pulling up the soft keyboard, and switching between apps all involve some delay. This is not true on the G2. It's 2.26 GHz CPU packs four cores and moves through day-to-day tasks without a hiccup. I'm not saying there's never any lag, but it is much more rare and much less painful. With the Sensation, lag was expected and smoothness was the exception. The G2 is the complete opposite: lag is very rare, and doesn't slow you down much at all.
*LTE: This is going to be pretty specific to T-Mobile US. I have been dying to get an LTE phone since T-Mobile launched it. HSPA+ is pretty good, I'll admit, but who doesn't want faster and better? I can confirm that LTE is indeed faster than the HSPA+ I experienced on the Senny. Of course, you have to account for the fact that the Sensation only supports 21Mb, while 42Mb is possible on HSPA+.
Coincidentally I got 39.99Mb on the SpeedTest app, which makes me think that it is throttled. It seems rather unlikely to get almost exactly a round 40 randomly. The LG G2 is capable of T-Mobile's new Wideband LTE which promises up to 150Mb, and I do live in a Wideband market. I'm not sure if my account is throttled due to being prepaid, or maybe the specific area I tested isn't upgraded to Wideband yet. I will certainly update once I have more experience with this phone.
In real-world use (e.g. downloading from the Play Store), LTE actually doesn't seem any faster than the Senny: I get about 20 Mb (2.5 MB/s). Still, latency (AKA lag) on LTE is significantly lower than on HSPA+, and the connection seems less likely to stall.
On a side note, each speed test takes up 70MB! of data. I would suggest anyone who is on a limited plan to use them sparingly. Thank goodness my plan gives me 5 gigs.
*Battery: This is another major reason I picked the G2 over the One and S4. It packs a 3,000 mAh non-removable battery. This is the first phone I've ever owned without a swappable battery, and that's because it's one of the few that meets my standard for such a device: If the battery is spectacular, I don't need it to be removable. You can read the praises of the G2's battery life from one end of the web to the other. I'll just share my experience thus far, which is admittedly limited.
Day one: Took off charger at 4pm (initial charge). Plugged in at midnight. 8 hours total on battery, 6 hours on LTE and 2 on Wi-Fi. One firmware update downloaded and installed. All apps updated to latest version on LTE. 3 hours screen on time. Battery left at midnight: 60%!
Day two (today): A little over 10 hours on battery. 4 on Wi-Fi, 6 on LTE. Pretty typical usage. Some texting, light web browsing, checking Facebook, Words With Friends, etc. Screen on time 1 hour 21 minutes. Battery left: 83%!
By this point, even with my standard battery saving measures on the Senny (Wi-Fi sleep activated, mobile data disabled except when manually activated) I would easily be down to 65%. The G2, with Wi-Fi sleep disabled and mobile data always on I am doing much better. 6+ hours of screen time is attainable on the G2 without any special measures! Getting much more than 4 on the Senny is practically impossible no matter what you do. I have seen users on the G2 forum who can get
In conclusion, you will be in complete awe if you move from the Sensation to the G2.