A third option is a 2-pack automotive clear used by automotive paint and body shops. Most of the stuff (Dupont Chromaclear, PPG, Glasurite, etc) come with huge price tags and are therefore impractical for the hobbiest. I have shot model cars with Chromaclear leftovers and that stuff is awesome.

I was in my local Napa Autoparts store the other day and noticed a reasonably priced professional clear coat system called MARTIN SENOUR PAINTS. I have no experience with the stuff but maybe you could look at it and tell us? Realize that a 2-pack system reacts with a catalyst and can be a bit heavier than a pure solvent based system. Also, the fumes are quite toxic but it is as durable a finish as you will ever get and will not turn yellow. Here's a link if you want to pursue it as an option:[link]http://www.martinsenour-autopaint.com[/link]

EDIT: Look for the "Reference Materials" button on the home page in the link and scroll down to MSDS and PDS for safety and product information.