I've used one of the Ultra-7 freewheels for years, and it's quite nice, but I have to admit it's "tetchy" in the shifting department -- it's quite easy to overshift when you're in a hurry to do so. So my recommendation for max. reliability would be to use a 5-speed at 120 OLD, or a 6 at 126.
The only difference between the wheels would be how they're built: a 126 OLD wheel would have an extra spacer on the right side, and would be built slightly more dished than one for 120. Hubs, rims, spokes would all be the same. The wheelset you put up the link to is for 5- or 6-speed, so probably built to 126. Some people claim that they can add some washers and a 6-speed freewheel to a 120 OLD wheel, but I've never tried it. As another poster said, it's cake to fit a 126 wheel into a frame with 120 rear spacing, though I imagine you might curse a little when out on the road trying to fix a f*** and reinstall the wheel with only two hands while keeping the chain out of the dirt too. Then it's nice to have the wheel fall right in.