Not much difference between the two, although if you're running FontReserve clients, it may be wise to go with FontReserve Server -- and vice versa for Suitcase. I personally use Suitcase client on my machine (I have no need for a font server yet) and find it runs decently well (this is also an older, slower machine) under Tiger.
They both exist because Extensis had their own software, "Suitcase," initially, then purchased the comany that produced FontReserve. It wouldn't be out of line to believe that the two will be merged in the future, or that the two are taken in different directions so they aren't so similar. Extensis just hasn't done anything with the FontReserve software yet.