I'd get something cheap like an ASG SAS 12 unless you've used shotguns before and know you like them, plus you play a lot of CQB(even then most, if not all gas shotguns are out of your price range). I had a TM M870 Breacher and it was a good gun but didn't get used much as I didn't play at sites very often where it would even be better than a pistol overall.
The advantage of gas shotguns is they are very easy to cock, the downside is the gas is finite so you need extra shells AND gas magazines if you get through a lot of ammo.