Stock SMS for me. I used to use Handcent but the more you use it, the more it becomes bloatware. But with TBOLTs memory it dont matter, really; more then enough to handle any kind of bloatware.
Stock just seems better. Handcent now looks more like eye candy.
On froyo I used Handcent only because I was having issues (like others) where some random SMS would be sent to the wrong person. I usually use stock, but now I am on MIUI, so i am using MIUI stock SMS.
Unless there a way to change the default layout. CHOMP, GO, and Handcent all have problem where when phone is sideways, full keyboard, you have no "Send" button. Until you go back or turn phone vertical. Unfortunately this is a deal breaker for me. Stock SMS DOES have the send.