The firm will beat in slightly quicker as well as skip a bit more. Medium has slightly more grip and durability. I personally prefer medium for all weather situations. If it gets really cold, I like some in the soft blend as they are even less prone to skips. If you just get one, I would say go for a medium.
If you are interested in getting a couple Vibram discs, go to communitydiscs.com. Justin is running a sale for the month of February on "Fugly" Vibram discs (buy two get one free). He has amazing prices and unsurpassed customer service. If you do go there, just make sure to click on the "Fugly Vibrams" link on the home page to be taken to the discs that are on sale.