Well, Diablo is a revision of OS2008, 4.1 instead of 4.0; I think a lot of the beta (in your words) quality of Chinook was because of the 3.0-4.0 jump.
I'd expect it to be substantially smoother, with no big surprises.
But, hey, we don't know yet, Texrat will only tell us "hee-hee-hee", and if the iPod wins by a landslide, Diablo probably wouldn't change your mind anyway. But if you're left teetering on the brink at the end of the month, then think about waiting.