I'd just like to point out that singletapping and alternating are not exclusive, many players including myself incorporate both elements into their playstyle. An obvious tenet of alternating that I'm surprised no one has mentioned is this: lots of notes are harder to alternate than lots of sliders. So typically, "hybrid" players will alternate on slider-based patterns (kick sliders, 1/4 slider repeats, anything with 1/4 gaps and such, or maybe just lots of sliders in any snap). Point being, someone earlier said not to learn them at the same time, but really it's ok to learn them at the same time if you're planning on or are used to using them at the same time.
On a sidenote, I have a habit of holding down my index finger whenever I had to use it, even if I can let go (I singletap with middle finger). For example:
If there is a 1/4 triple with the last note leading into a slider, a normal person would do zxz, and the z would be the only button held. However, if you look at my replays, you would see that I'd have both z and x held down, I'd be holding the z down consciously for the slider, but my index finger subconsciously keeps the x held down. This happens for all kinds of 1/4 into sliders for me, like if there's any size stream into any type of slider of any length, you'll always see my K2 held down.
You'd normally say that it wasn't a problem, but let me tell you this: I'm pretty sure it is.
I can't do tons of triples without awful accuracy, or a miss. Granted, that mapping technique is horrid, but calling out a bad mapping pattern won't get you the score. Let me explain why exactly I can't (according to how I think it is):
My index finger can't immediately let go of the button after pressing, let's say it holds it for an extra 1/4 length. For a single triple or into a slider, that is fine. When there are many triples, they are typically spaced by a 1/2 gap between each other. Because of the extra 1/4 my finger holds x down though, my fingers always end up playing triples as a continuous 1/4 stream. There are 2 points as to how this would fuck up my gameplay:
1) Since I do triples zxz zxz, but my index finger can't handle 1/4 presses, I'd end up getting out of sync and completely messing up my rhythm (typically, when this happens, I still end up streaming as if it were 1/4 even after the stream ends and it's 1/2)
2) My mouse movement is seperate from my pressing, so it moves as if my fingers could probably do triples. But because I'd be playing it like a continuous stream, when my mouse moves to the next triple during the 1/2 gap, my fingers are still pressing, so I end up playing the first note of the next triple a 1/4 gap early, which of course messes up the entire triple for me.
Well I wasn't anticipating writing an essay on my flaws, but ok.