I second what Ron said. But more important I think is the fit. A suit that fits you perfectly will keep you much warmer than one that almost fits. Obviously, different manufcatures have slight variances in their cuts, so it's probaly best to go and actually try on the suits. For example, I purchased a 3mm shorti which is basically brandless, it was the stores special, which i bought for about $50, and it works great. I've gone diving in 75 degree water and been quite comfortable in it.
However another point to consider is that you can alway let water in to cool you down in a thicker suit. The flip side to this is that you will be more bouyancy swings, and will require more weight at the beginning of the dive.