Front mounted. If you need a winch to go backward it means a couple of things have gone wrong in my opinion. First, you are alone, as if there was a second truck with you it can be assumed that if it's in front it already cleared the obstacle and is now a winch anchor or it's behind you and can snatch (or winch) you out backward. Second, you've pointed your truck inappropriately with intent to get stuck (i.e. into a big mudhole to see how far you can get) in which case the first concern really comes into play. I can see having the redundancy of a second winch out back, but I don't believe the extra weight is really worth the advantage.
As for the receiver mounted winches they look really great initially and I think they have a place in a utility/farm truck, but not an expedition rig. My primary concern would be trying to haul a 90+lbs winch and cradle into a slippery mudhole, then inserting it and plugging it in without ending up injured. Up here in the Northwest you can end up on a trail where most of the front bumper is buried. This would be less than fun to insert a cradle into what amounts to a hitch, especially in the rain/dark/snow. Not really practical in those conditions IMO. Further, the extra length of the mounting assembly equals extra leverage on the mount on anything other than a straight pull which could lead to a bent reciever or worse bent chassis. In the rear most SUV hitches are rated well below a 9000lbs load, let alone double lined so the capacity seems suspect at best.
I'm with Steve on this one, going forward is the objective.
Marc