I've been working on pretty much the same decision, either a 40" LCD or a 50" SXRD(Sony's DLP basically). I'm comparing 2 Sony models right now, the KDL40S2400 and KDS50A2000.
I'm leaning towards LCD now becuase you don't have to worry about replacing lamps, much wider viewing angle, and from my personal opinion I think they have better picture quality.
On the other hand, with SXRD you get a bigger screen, 1080p, HDTV tuner built-in, faster 2.5ms response time, and although I think LCD looks a little better, it's not much.
Don't really know if it helps you or not, and after typing that I find myself almost leaning for SXRD now...that's why I'm waiting till Thanksgiving/Christmas time, hopefully I'll make my mind up by then, or I'll catch a really good deal I can't pass up on.