The 22mm Serpent Mini was the first tank I really realized you don't need wide open airflow to have a good vape. It was my favorite RTA for some time. The Merlin and Merlin Mini are both great RTAs too. But I definitely think the Siren beats it. The Merlin Mini might have slightly, and I do mean slightly, better flavor. Maybe, but it's definitely competitive. But I like the airflow, building on, and juice capacity on the Siren more than I do on any of those other RTAs.
And yes, I'd say go for the 22mm Siren. The 25's window is a gimmick and basically useless imo. That alone wouldn't be a deal breaker if you were after a 25mm RTA, but the problem is that it's press fit, so it's not exactly built to last. I've heard too many stories of 25's window getting leaky or the window just plain falling out after a few months.
Also, I think I mentioned it in the thread, but I'll mention it again. Tighten down the positive post or else it'll weep a bit. After tightening down the positive post I haven't had any weeping to really speak of. For some reason or another Digiflavor must not be tightening down the positive pin post screw very well, so both my Sirens weeped slightly until I fixed it.