FX Revolution or

What Mark said but in the absence of more info and with the choice limited to just those two, the Revolution is clearly the better air gun. If you are expecting it to be good in any particular way and have the funds, that would be the one I'd choose. 

My general thought on semi-auto is that they have great potential for novelty and a few fun afternoons of wasting pellets before that novelty disappears. I'd want to maximize the potential for fun during that time.

I couldn't see myself ever wanting semi-auto on a serious hunting rifle or target gun, so I wouldn't want to spend a lot of money on one. I wouldn't want the absolute cheapest either as every cheap semi auto air gun I ever owned broke after a few months of heavy use. 

If I wasn't limited by the constraints of those two guns and .177, I'd go with the Air Ordinance SMG 22. You can rapid fire for hundreds of shots without reloading air or pellets. Nothing spoils rapid fire fun more than small pellet or air capacity. Plus, it has both full and semi-auto.

I'd be lying if I said I wasn't curious what a full auto spray of polymags would do to a squirrel...