I would think that would have a lot to do with the availability in the marina you are considering. If you are in an area with multiple marinas, have them make an offer and choose what fits you best. I am sure marinas know who there competition is......
It will depend on the location, anything north of Maryland and I think you will find a shortage of slips due to Sandy and they will be at a premium. South of Georgia is "in season" and most marinas are filling up and on reservations.
I thought the same thing when shopping for three slips. Unfortunately I am in Chicago where slips are at a premium. I found a marina 27 miles away that was only half full that gave a 1/2 discount on my second slip. I ended up with two wells all to myself.
The only downside to having two boats is cleaning two boats!