It's a toss up. If it were a ~20k mile difference, I'd take the V8 all day. Being as it is, I could be enticed by the overall mileage of the V6.
I'd climb under both and look at rust. It's possible that the V8 has a better body underneath than the V6, even with lower miles.
I can't find the thread, so I'll add a few things here...
I'd drive and listen for:
grinding/droning when turning. (wheel bearings)
thumps, clunks (suspension components)
drive slowly (foot off the brake, no accelerator) in the V8 and listen for 'popcorn' type noises with the driver's door open (transfer case viscous coupler)
climb under and see if you can move the driveshafts at all (laterally) and if so, will probably be wearing out soon.
Look for leaks- trans, rear main, rear diff, TC, front axle, CV's, radiator, valve covers, etc.
Try the AC. If it cycles every 3-4 seconds but blows warm, it's either low or leaking. If it doesn't run, either the switch isn't cycling it because it's empty, or it's done. Make sure it isn't bypassed with a shorter belt.
rust- look for the rockers behind the rocker, under the rear door, wheel wells, and door seams.
Get up to speed and do some sort-of panic stops. Check brakes, does it track straight, and listen for the transmission downshifts.
Is there any stuttering when letting the wheel turn back to center? (dying pump or air in system)
See if the V8 has Motorcraft plug wires. If not, they've been changed. If so, they're probably original if they don't look decently new. Reach through the front well and pull a wire, check to see if the plugs look new. A sign of a well maintained truck in most cases.
Open the door and lift up the bottom. Play? Needs new door pins.
Check shocks- are they rusted out? Original, need to be changed. The bounce test does not work well on the Ex.
Any interior parts, etc., missing?
Try every button, switch, door handle (in and out) for function. Does the rear wiper work? Does the rear defroster work? Turn it on, go back, and breathe on it and see if it clears by the wire.
All of that stuff adds up if you have to replace it. All possible here for help and how-to's, but $$ adds up if you have too much to do.
Try this too- get a dirt patch, road, parking lot, and floor the V8. It should sense slip and engage. Do you see evidence of 4 wheels spinning if you look back?
Does the 4W-LOW engage in the V6? (Put in neutral, foot on the brake, then go from hi to low, same to disengage) Try it out.
The normal stuff- that comparo is a toss up for me, I'd rather have the V8 if it sounds good and drives straight, I just think it's a more stout drivetrain. JMHO...
Do you have any pics of these trucks?
1991-1997 had this hatch style:
and this front (including 1998, some had foglights also):
1998-2001 had this one:
and this front '99-'01
The 'Limited" had a special grille, as did the Mounty, with both a different grille, and then the 1998+ Mountaineers having a different front end look altogether:
A Mountaineer will be the same. Aside from that, and aside from some differences in standard/optional features (check wikipedia) they are pretty much the same.