The 5750 that I have also tends to cook itself without third-party fan control. My best friend has the same problems with his ATi software (we both run Win7 Ultimate 64 bit, it's probably Specific).
ATI just runs hot, it isn't Win7's fault. Use CCC to manualy control the fan, it can do that. Not that I've had any temp concerns (using Win7 Ultimate 64-bit).
Anyway, I've used a few ATI and nVidia cards, all low end (sub $150 range) and so far the drivers are not bad at all for either side, in this aspect I really don't see a difference, they both have problems now and again.
A big pro I have for ATI is on-board and dedicated vid cards working together in non-crossfire. I have a 4290 on my motherboard and the above stated 5570. They run off the same driver. As a result I can have 2 monitors on my 4290 and 2 on my 5570, run youtube in 1080 on the 4290 and not have it effect my game on the 5570. (nVidia might do this too, but I haven't seen it and onboard nVidia seems rare now)
Another pro for both sides would be if your dedicated card dies and your onboard uses the same driver, you don't have to go through 800x600 hell to install its driver while you get a new card just to install your old driver back again.
For those who cant understand ATI's naming, its really not that hard. I have a HD 5570, lets break it down:
HD (the series name)
5 (current generation of card, higher = newer)
5 (class of card, 5 is probably low-mid range, higher number = higher class)
7 (level of the class, higher = stronger/faster)
0 (because everything is more awesome with a 0 on the end)
To this end, I have a 5th generation HD series in the strong low-end class.