I think it helps to understand your objective. In my mind if you are capturing what the sound guy puts out, that's "taping." If you are recording all the raw feeds and remixing them that's "multitracking". The significant difference isn't just in the recorder, it's what you do with it in post. If you are friends with a band with 2 guys with acoustic guitars, then 2 vocal mics + 2 guitars + 2 room mics really is a complete multitrack in 6 channels. If you grab all the individual feeds for a full rock band you are talking 16-24 tracks.
I run 4 channels (2 SBD + 2 AUD) 80% of the time, and R4/R44, etc is perfect for that. I have an HD24 setup which I use 10% of the time, and probably the other 10% is just 2 track AUD.
To me, 2 SBD + 2 AUD makes a lot of sense. If I'm getting a mono SBD feed, sometimes I take the spare channel and grab the bass guitar DI. I have friends with 680's who will run 2 AUD + 2 SBD, and then grab a couple of individual channels (like lead singer's vocal and kick) to fill in the weak channels. If this kind of thing "a couple of extra channels" appeals to you, then something like a 680 is great. Speaking only for myself... I'm happy with 4 track, or I want all the stage mics + AUD, which is where I use the HD24.
Does that make sense?