Keeping in mind that one persons warm suit, may be someone elses "doesn't work for me" suit, the general key factors are:
1. Fit. The better a suit fits the warmer it will be.
2. Material. Stretchy material makes if fit better, but compresses more a depth. So what makes it warmer shallow, makes it cold deep.
3. Your personal cold resistance.
4. The type of diving you are doing (active, swimming a lot, to taking pictures and not moving).
I've been in 38 degree water with an oneal j suit (no longer being mfg'ed) and been warm, and was cold this last week in 51 degree water (but was a lot deeper)...
Any semi-dry will be warmer than a standard suit, but with a standard suit you can wear a skin to make it a bit warmer, and with a semi-dry that does not work.
If I were to make a general recommendation, assuming the diving is not going to be that cold, would suggest the best fitting (and that will take some trying on to know) 5mm you can find.
Add nice boots, a hooded vest, some good 3mm gloves, and you should be covered for a fairly wide range.
The other option is a two piece 3mm, which is not as warm, but can be used in different combinations.
I just got back from BVI, where the water was 78...taking pictures, that is the full two piece three mil for me.. was warm, but any heavy swimming and I was too warm.
For the record, I own a lot of suits... Bare, Pinnacle and Henderson to name a few. Pinnacle happens to make a suit that is a near perfect fit for me, so I own three different thickness of Pinnacle. The wool does very little to actually keep you warmer, except slowing down water movement...nothing beats a good fit.