Who to dive with is not a big consideration, as all three shops in Varadero are affiliated, and the workers move back and forth on a daily basis.
Prices improve with quantity, so buy the max (10 dives I think).
One little note of caution, I was in Varadero from Oct.20th to Nov.10th, when the "Nortes" started to kick up. The visibility turned to poop, and diving was canceled 3 out of 5 days. On ugly days they will bus you (2 hours each way) to Bahia Cochinos on the south coast, nice diving but not worth a second visit to same exact spot. I believe the time you are going is around when the Nortes are abating, but...
One big note of caution, the rental equip is pretty scary.
Sorry I can't offer any advice on lodging.