It seems like the Wiz screen issue is rare, and the OLED really does look nice. Turn on the auto-off feature in the settings and you won't have to worry about burn-in, which shouldn't be an issue after a few hours of gameplay anyway. If anything the issue with the Wiz as designed was the shallow d-pad with oversensitive diagonals, which bothered me at first but feels so natural to me now that I don't notice it at all.
Again, I say this without experience with the Caanoo (which I'll toy with this afternoon). But the wiz is great. Awesome emulation on it is a reality, not a probability. I'm not buying my Caanoo out of dissatisfaction with the wiz (or with my F100 MK II). Quite the opposite.
If you just want point-blank advice, considering the concerns you've laid out, I'd say get a thinkgeek wiz at the awesome $100 price tag.