I think it will be hard for people to answer with one or the other.Both has to be an option too.
I primarily am a front collector,but try to add a wide variety of backs into the mix also,so I can end up with a very enjoyable mixture of front/back combo's.
When I look for a new card,my first focus is always who is on the front.When I decide on a player,I check around to see what front/back combos are available(within my price range).If there are,say,a Nap Lajoie w/bat Piedmont and another Nap Lajoie w/bat Polar Bear,within a few $$ of each other,I'd be more inclined to go for the Polar Bear.
I love the ultra rare backs-just can't afford them right now
Regards,Clayton