Oil based polyurethane is yellow to begin with and turns more yellow with age. I've never used water-based poly so no comment on it.

I like epoxy clears or automotive clear (2-part). Both are excellent, very fuel proof and don't yellow (at least for as long as my planes live which sometimes isn't very long).

But one thing that is very important is that the finish under the clear can be put over the clear without a bad reaction. For example, I once applied a silk and dope finish to a model and then painted using Perfect brand polyurethane. I had to apply a patch some time later and the dope crazed the poly. I kept applying dope and sanding until the crazing stopped which was several coats.

Now I only use one type of paint start to finish. If I'm going to overcoat with epoxy I begin with epoxy. If I need to apply the cloth with dope then I paint with dope.

Also be sure to test the clear coat with a spare decal to ensure it won't destroy the decal. I've never had polyurethane react with a decal, but as I said, the stuff I've used was yellow so I don't use it for that reason any more.