As was mentioned above, 4x4s have a reduced tow/load capability (~450-500 lbs to allow for the weight of the components), additional maintenance requirements and complexity. Unless you live in an area that has need of 4 wheel drive or have a farm where your TV does double duty, or, camp in areas that are muddy, sandy or icy, you can save money and load capacity by staying 2x4. If you buy a 4x4, 99.99% of the time you will be in 2x4 mode, why pay the premium unless you have a specific need?
That being said, I have towed with a 2x4 and 4x4 and will say that a full time constant 4 wheel drive is much more stable and secure. The only vehicles made that have that feature are the Jeep line of SUVs. You will find a "hard core" group of us Jeep TV owners that swear by the "full time, all the time" advantages of 4 wheel drive. The Jeeps (Commander, now discontinued, and Grand Cherokees) make excellent tow vehicles for the small to medium sized family of trailers (the Commander and GC have tow capacities to 7400 lbs) and go from pavement to gravel to mud to snow to sand to rain slicked roads without missing a beat. I too am in the mountainous area of PA and all of my vehicles (even my lawn tractor) are 4 wheel drive, but, that is not because of towing, but, just a result of my environment.