340 or 383 for 1968 Dart

I've had both in relatively stock form. They're pretty evenly matched, but if you forced me to choose, and I was starting from scratch, I'd go with a 340. Although around here, at least, a 383 is a lot easier and cheaper to find and buy than a ridiculously priced 340 block that may or may not be complete or any good to begin with...
If you've already got one or the other sitting around, use that and don't look back.