256mb will make quite a big difference, the stock sensation Rom is a lot more laggy than the stock amaze Rom is, and they're practically the same thing. If not less "stuff" on the sensation.
So unless you can't handle not flashing a Rom every day, get the amaze.
Even with underclocking to 1.2ghz my phone is still smooth, and you get the battery life of the sensation.
Now of course, everyone here will be biased towards the amaze. And if you ask the sensation people they'd probably say the sensation.
It all comes down to your personal taste, I was in the same position as you. But I chose the Amaze, and I don't regret it at all.
If you can, go into a T-Mobile store and test them out.
Sent from my HTC Amaze 4G using XDA