It all depends on your type of diving. Here in the UK, your feet would drop off if you wore full foot fins, as the water can get quite nippy!! As such, most of us are in drysuits with open heel fins.
Open heel fins are generally more comfortable, because you also wear a wetsuit/drysuit boot with them. Full foot fins have to be a good fit otherwise they rub, although wearing socks can help with this.
When I am in the pool with students, I always wear full foot fins, because otherwise the boots make your feet too bouyant when demonstrating skills.
I think (at least in the UK) there is also a much wider choice of open foot fins, because they do not have to make as many sizes, therefore there are more models of fin.
Hope this helps