Although I never owned a Gen 1 ST or a V6 Gen 2 I did have several supercab Rangers with the 4.0L V6. The Gen 1 ST was actually just a different body on the Ranger frame.
Given a choice I would say go with the V8 on the Gen 2. As you said, there is little difference in fuel mileage and you will gain a much higher towing capability. Plus with the right exhaust it just plain sounds better.
On the V8 you will want to look for a truck that has had the transmission reprogrammed by a Ford dealer to correct any shifting issues. You should also find out if there have been any exhaust manifold leaks and whether or not those have been fixed.
Those are the major issues with the V8 drivetrain that most owners have had to deal with. Also, the 2007 and probably some early 2008 V8s had the two-piece Motorcraft spark plugs. They can be a pain to replace.
Most other issues with the Gen 2, like front wheel bearings, radiators, etc., I believe are common to both the V6 and V8.
