To throw another side to the argument, I recently took my daughter to the Peadiatrician who asked me if my daughter was on formula or cows milk (she is now 16 months). When I advised cows milk she said this was good as formula was not great for their teeth. Not sure if this is true or not.
Another thing to consider is how much travel you do. We switched to cows milk once my daughter was 12 months old. We have done a lot of travel and often been caught out in flight and in hotels who are unable to provide adequate cows milk. We would not have had this problem if we could take formula and just mix with water in flight or in a hotel room.
In saying this though, we do prefer cows milk, it is easy, convenient to obtain (when at home), no measuring or mixing and not fussed when there is wasteage (it is cheaper than forumla). Good luck, but if you are not fussed about the other factors, I would stick with cows milk as opposed to doing two transitions.