As Texan stated, you'll get LOTS of opinions on this question... here's mine.
I have a one-piece Roadcrafter, have had for 10 yrs since I lived in SoCal...
I was debating the 1-pce vs 2-pce when I bought. If you're looking specifically at the 2-pce roadcrafter, the pants don't really hold up by themselves so you either leave them zipped to the jacket or wear suspenders or something.
When i was considering, I ran into a guy at the BMW dealer in Ventura, who explained that with a 1-pce, you never had the option of leaving the pants on a hook in the garage because "it was just a short ride" - and remember, you gotta save your ass, that's where the skin grafts come from.
I never found the 1-pce Roadcrafter too bad for heat as long as I was moving, even in the summer in LA.
If I was to buy another, I'd probably go for the 1-pce again, or else a Darien jacket and pants (better in the rain, and a bit more comfy). I THINK i'm old and wise enough now to never ride with just the jacket.
YMMV, opinions are like ********... etc, etc, etc
Hope this helps
Griff