I find that most people want to wear open-heel fins for several reasons:
1) Your LDS is correct - open-heel fins are in general bigger, stiffer (always good things <grin>) and more efficient.
2) When you are not diving (e.g. walking around the boat, across the rocky shore, back to your car) you are going to be MUCH more comfortable wearing booties.
3) Full-foot fins almost always end up trapping sand between the fin and your skin - this leads to blisters, sores and general unhappiness
4) A good set of open-heel fins will work in both warm and cold water, with any thickness of boot. If you buy full-foot fins, then you end up with a set of open-heel fins anyway.
Ask yourself this - in what way are the full-foot fins going to be better?