ok, beware, serious post
as i'm full-alternating here's my insight on the topic:
I think alternating is the best way to start playing the game.
Reasoning is the following:
You instantly learn to alternate some patterns that you could easily single tap at easier songs but it will become impossibru at insanes/extras with higher bpm like triples, consecutive fast doubletaps etcpp
From what I've tried(I occasionally start singletapping with any of my both alternating fingers) it is relatively easy to go back to singletapping if you started playing the game alternating. My accuracy doesn't suffer at all(unless the map is fast or consists of triplespam and i try to singletap the triples) and 70% of my skill is perception, knowledge and aim anyway and it lies within my mouse hand and my eyes. And I'd highly doubt it's different for any player around my casual skilllevel.
tl;dr As alternating is mainly a mental problem(due to different reasons) it's easier to go from alternating to singletapping than the other way around. Alternating players don't have a general problem with singletapping it's just more relaxed to alternate when they can already do it.
The main thing that's really different i noticed so far is reaction on hitsounds and in that context your consistency.
With singletapping the feedback of hitsounds is absolutely clear and a conscious reaction will lead to better results which isn't a necessity at all when alternating.
Tbh I would love to have two hit error bars, one for each key I'm using ._."