I'd say post Graphics Update the gap is more like 10-20% - though of course it all depends on the hardware and the title.
I play SC2 in both OSX and Win7. Performance is better in Win7, but I use v-sync to cap framerates and play with medium-ish settings so the performance difference is not relevant.
I keep my Win7 partition around for gaming. If you have already gone to the trouble of setting it up, and don't need the HD space, you may as well leave it there for now.
As to the differences.
0) Performance gap, as noted. Doesn't affect play at all unless your hardware is really ancient/limited.
1) Sometimes there are sc2 helper apps which are Win only. To date though all the ones I have used are either x-platform or have OSX workarounds eg. sc2gears and mapcraft.
2) Mouse acceleration. "Serious" gamers have this off. You can turn it off in Windows, you can't in OSX. (Ok, in fact you can, by using USB Overdrive or SteerMouse (I use USB Overdrive). But it's more straightforward in WIndows.)
3) Maps. In Windows you can load a map by dragging the map icon onto the SC2 application icon. In OSX you have to load up the map via the Editor or from within the SC2 game itself. (Last I checked anyway.) Tedious if you do this a lot.
That's everything I can think of right now.
Edit: Oh yeah, temperatures.
Ok so, temperature management under Bootcamp Windows is not very good. If you just use Windows for Word or something, no issue. If you use it for heavy gaming, could be. The consideration here is how long you want your hardware to last and/or how valuable your data is. The most heat-vulnerable component in my iMac is the HD. Opinions vary but speaking generally, sustained high temps reduce the lifespan of your HD. Over 50C isn't great, over 55C is bad, particularly if it happens regularly for long periods (eg. "serious" gaming).
There isn't any fantastic works-on-all-Mac-configurations heat solution for Windows on Macs yet. But there are a number of "good enough" options.
In my case (iMac i7), I run Temperature Monitor and iStat under OSX to keep an eye on temperature. I also have smcfancontrol installed. Before gaming, I set fans with smcfancontrol to a moderately fast speed. I then reboot into Windows. As long as you reboot and do not shut down and start up, the fan settings stay at the custom speed, through my Windows gaming session.
After I finish gaming I reboot back into OSX and just check the temps. If my HD in particular is in the mid 50s, I up my preset "Windows speed" setting in smcfancontrol a bit.
The reason I monitor and adjust presets from time to time is because different games can produce different amounts of heat, and because ambient temperature is a factor and where I am summer is very hot and winter is pretty cold.
Note, if you don't do a lot of gaming, you can probably completely ignore the whole heat issue and just use Windows normally.
Final note, there are some Windows apps which monitor temps, and some which allow you to set fan speeds as well. Unfortunately, there doesn't seem to be any app yet which does this reliably on an iMac. There is or was an app which set speeds correctly from Windows on Macbooks/MBPs but not other macs. There are Windows apps but they seem to either not see or not see all of the temperature monitors or fans, or report incorrectly. For me, smcfancontrol from the OSX side has been the simplest option.
One final note about SC2, as you may be aware, the frame rate can skyrocket on powerful hardware. Some people say this doesn't = more heat but it seems to with my machine. I don't need more than 60fps anyway so I set v-sync on under the video options to cap it, under both OSX and Windows.
glgl!
