I have the Serpent Mini 25 and the Merlin Mini, never used the 22mm Serpent Mini.
Of the two that I have I'd have to go with the Serpent, without question, in nearly every department. The only thing I like better on the Merlin is the design of the single coil deck.
The Serpent holds twice as much juice. The airflow, while comparable, is much smoother on the Serpent in my experience. I have two Merlin Minis and they're both noisy even when they're not whistling, which they do unless the airflow is wide open with the dual coil deck in. Close it off at all with either deck(not counting restrictor screws, just AFC) and it gets pretty annoying.
With identical builds, dual or single coil, the Serpent has better flavor, probably due to the smaller deck(16mm versus 18).
Due to the size and design of the single coil deck on the Merlin you can run a massive coil in it. If you plan to run a single coil build on a dual battery mod, that'd probably be the better option. I'm sure you can get great flavor from it, you just have to build big enough to fill that deck. I got it for my VT75 Nano, which I try to stay at 50w or under with, and I'm having trouble getting a flavorful vape at those power levels.
Also, you can toss some pretty respectable clouds with the Merlin's huge dual coil deck as long as you don't mind constantly refilling.
They both have their pros and cons, but in my opinion the Serpent Mini 25 is the better of the two. Like I said though, I can't speak on the 22mm version.