I had the feeling that the Nice Ironman was much harder than the UK one. That was confirmed by some posts on the Runner's World forum.
However, looking at the facts, it's not that obvious. Surprisingly, there's more ascent on the UK one, for both the bike and the marathon. I thought the nice backcountry would be more hilly. On the other hand, the temperature in Nice will be much higher, particularly on the run, well exposed to the sun. As for the swim, the waves in Nice maybe a bit of a problem, but that should be compensated by the higher water temperature...

 UK IronmanFrance Ironman
General LocationSherborne Castle, DorsetNice, Alpes-Maritimes
DateAugust 2008 ?22nd June 2008
Cost?330+10+40 = 380€
Laps22 with a land exit in between
StartIn water mass start6 starts sections
Time limit2 hours 20 min2 hours 15 min ?
EnvironmentLancelot Brown Lake (15.5°C)Mediterranean Sea (24°C)
Time limit10 hours 30 min10 hours 45 min
Water points9 ?8
EnvironmentEnglish countrysideSouthern France backcountry - may be warm
Laps2+2 ?4
Ascent773mTotally flat
Time limit17 hours16 hours
Water points18 ?24
EnvironmentEnglish countrysideSeafront (Promenade des Anglais) - may be hot

Overall, the website of the Nice Ironman is much better than the UK one. I know it may not mean anything about the actual race, but it gives a bad impression...

Ultra choice to make.