I've been advised that to achieve proper trim, it's better to have as much of the weight on one's back as possible and as little on the waist as possible.
This may be true for your setup, but isn't always. For example, folks diving wet here in the PNW often find they are very feet-light because of all the neoprene on their legs. The right place to put weight is where it works.
To the OP -- a wing in the 30 lb range will work for warm and cold water. It's a little bigger than you need for warm water, but still small enough not to taco badly and to remain easy to vent. I dove my 32 lb wing in the tropics in a wetsuit for the couple of trips we did where I was still silly enough to use thin wetsuits, and it worked fine.