Walter,
What fabric are you using? My answer would change depending on what you use, not from a durability standpoint, because I think it's a toss-up, but from an "effectiveness of sealing" perspective. If you're using the economy fabric, I believe you'll get a much better seal in fewer coats with less hastle using spar varnish from the local BBS. If you're going with the premium fabric, I'd go the paint route. I did two painted boats with the economy and one of them still has pinholes somewhere and the other took spray on fill primer to get a bunch of spots. With the varnished one, three coats on the bottom, two on the top - good to go and doesn't leak a drop. The main thing with the varnish is to apply it as sparingly as possible at first so you don't get runs and drips that go through, because every application error will be visible from both directions...
Hope this helps!