You can put varnish
on, overtop of epoxy. In fact, in anyplace exposed to UV, it's kind of mandatory. So you needn't choose one over the other.
Go to, & or call, the System 3 Resins, & West System sites/tech help lines, & they'll recommend which (brands/types) of varnishes, to use with resin formulation X, or Y in order to get the best results.
MAS may have some as well, ditto on other resin makers.
I used to know, but that was ages ago. However, several of the varnishes recommended to me the last time I did it, weren't of the high gloss variety. So you should be good, regarding getting the finish that you like.
Also, there's a LOT of information in the application guides, & available online - both in text, & video format. Some of which is especially helpful in achieving a good finish. By preventing things like outgassing, which causes those pesky micro bubbles in clear finishes.
Look up Russell Brown, & PT Watercraft, his book & online videos are more than worth it, for what you're doing. LOTS of good tips.
Plus, if he was good enough with composites to get recruited to work on an America's Cup boat, amongst many, many other things...