I say Galaxy Nexus. It will be supported longer.
Here's my two cents as the owner of both (though I have now sold the Sensation....if that makes Sense).
The GN has a better camera. No doubt. Not only does it take faster pictures but it lets in much more light in darker surroundings and the colour balance is a lot better when using the LED flash. 10 out of 10 pictures will turn out better on the Galaxy Nexus. This is because HTC cameras are inferior to Samsung's on the whole. Megapixels only mean something if the camera is good enough to make them worth counting.
The Galaxy Nexus has better balance in the hand. The Sensation is made of lovely metal etc but is not so comfortable to hold in the hand. This is just my opinion though.
Screen resolution is noticeably better on the GN. Also screen size is a boon and colours pop, though not as unnaturally as on the Galaxy Sii. Viewing angles are also better.
Charging time is rapid on the Galaxy Nexus. It is the first Android phone I've owned that charges as fast as an iPhone. I can use the same charger as for my old Sii and it charges in less than half the time Great for quick top-up charges The Sensation isn't as slow to charge as the Sii but is nowhere near as fast as the Nexus.
No card slot on the Nexus, which means I can't use my 32gb MicroSD as I could with the Sensation. I tend to put my movies on a USB stick and watch them with a USB Host cable though so this isn't a major issue.
No dust issues. My Sensation went back three times with dust under the screen. I eventually sold it on because of it.Super Amoled phones are not at risk of this as the digitiser is tightly fused to the OLED panel.
Signal is MUCH better on the Galaxy Nexus I get never drop calls from my room on the Nexus where I would lose connection all the time on my Sensation (as well as my Incredible S). I also had a hard time getting wifi in my room with the Sensation and Incredible S.
The long-term future of the Galaxy Nexus is much brighter. It is guaranteed to get whatever updates may come over the next year whereas the Sensation is unlikely to get the next incarnation.
Both devices have pretty crummy speakers. This is something I have come to expect from my HTC devices but is a little disappointing on the Samsung as their devices usually have good speaker units. Could be a software issue.
These are my feelings. I have only had tge device for a few weeks but there is absolutely no way I would go back to a Sensation/XE, or in fact any current HTC. I really do love my Nexus, though there are parts of Sense I did like and miss.
