Hi,
I have both the Iphone4 by Verizon and the Sensation 4G by T-mobile. Honestly, the Sensation is a great phone FWIW but....... It's still an Android. Which means, you'll get hang ups, occasional reboots, and just not as reliable an OS as Apple.
Depending on where you live, T-mobile's coverage is not at good as Verizon in some states. On my recent travel, I brought both phones to Arizona, and was unable to get signal on T-mobile while my Iphone4 had 2 bars with Verizon so I was able to make an important call on. And to be very honest, Iphone has a more solid worryfree interface. Unlike apps on the HTC, it doesn't drain as much from your memory. In other words, consider the iphone as a military grade hardware whereas you can't brick it. I've had mines for close to a year without rebooting it and it still runs solid and stable whenever you need it.
The HTC Sensation 4G has a bigger screen and rivals the iphone in certain ways but its more a gadget then function. There are goods and bads on both phones but depending on your needs, you can't really beat an Iphone.
Iphone pros:
1. Speed is hardly an issue although the Sensaton has a dual-core (more of a marketing gimmick as you could hardly tell the difference.)
2. More reliable and much easier OS interface. (aka smartphone for dummies like me).
3. Apps are simple yet does what you expected. Can't say that about Android apps. (developers need to take heart).
Iphone cons:
1. Screen is not as big compared to Sensation 4G (so watching movies would be better on the Sensation 4G)
2. Battery won't last as long as the new Sensation 4G (maybe on the next gen but I highly doubt it.)
3. Can't open the back covers so no micro SD upgrades. (you get what you pay for only).
4. Still the suckiest contact, music, personal syncing interface on the planet via Itunes! (Google did an awesome job here!)
HTC Sensation 4G Pro:
1. Larger screen, higher resolution videos, latest so-called HIGH TECH phone! (this is one of the sellable features.)
2. Phone call clarity! (quite nice to be able to hear everything clearly)
3. Expandable memory options and better syncing via Google.
4. Way better looking interface when it works correctly. Additional widgets makes this phone feel like a tablet computer. (requires frequent app kills and monitoring).
5. Longer battery standby compared to the Iphone.
Cons:
1. If you mulitask, be prepare to kill apps upon apps or else you'll drain the battery and heat up the back of the phone.
2. still a little buggy as runs on underdeveloped Android 2.3 OS.
3. Be ready to experience random reboots if you kill the apps too quickly.
4. Be prepared for shitty signal receptions via T-mobile! (when it works, it works very well!)
5. Relearn the apps, and spend some more times reading and tweaking the apps.
So overall, if you want a worryfree smartphone, Iphone/Verizon would do you well. But if you want a So-called "State of the Arts" smartphone and get caught in the hype of dual-core, latest and greatest, try out the HTC Sensation 4G.. learn to use the kill apps and different methods of conserving your battery on the phone. Lastly, if I was givin an option to choose only 1 phone in a more remote area for both functionality and entertainment, I'd still stick with the Iphone TBH..