My opinion, having worn both on different occasions:

Both block wind completely, so there's some warmth gained right there.

Wetsuits only work when you're wet - they keep a layer of warm water against your skin. Jump in, let it fill, then get on with the race. Otherwise, you're just saving your sweat (which can't evaporate) for later. I like a wetsuit for high wind, cool water, but warm air temps (70's) when I'm crewing on a wet boat (like a H16 or Nacra 20). Wetsuits are pretty inexpensive.

Drysuits only keep you as warm as what you're wearing underneath - which is nice, because you can layer up or down depending upon conditions. If you're dry, you're not losing core temp to cold water. Most drysuits are "breathable," so you stay pretty comfortable. I wear a Gore-tex drysuit in a very wide variety of conditions as crew, and any time I'm skipper. Drysuits can be very expensive.

that's my $.02