Suggest you start in Lim.10
Single stack is a newer division and thus has fewer shooters in it. It's rules are more limited with holster choice, angle, location and gun coverage. Not to mention that you will be handicapped with 8 +1 rounds shooting stages that give the advantages to the limited or open shooters. Unless you're an exceptionally accurate and fast shooter, I'd recommend Limited 10, just to get you back into the game, re-learn some old and some new rules, gun familiarity and reloading skills. You are likely to have as much fun with that initially and then after that you can move to Single Stack with the holsters on or behind the hip as with the mags which you will have to carry quite a few. Main thing is to relearn the safety sequences, the newer rules, and familiarity with your pistol of choice. Welcome back and have fun!