I level on Master; Hard is a bit too dull. I think Hard is faster if you can rack up big enough killstreak multipliers though.
As for how many mats I have when I hit 70 - I honestly have no idea, does anyone even track that? "More than I'd have if I rifted to level instead and finished with zero" is probably the only count I can give you
I like leveling solo. I might give split leveling a try in S8 though, or roll with some friends who are interested in getting back into D3.
Not sure what you mean by "get enough stuff" - the rewards are individual whether you are solo or split-running, the only difference is that split clears are obviously faster (if you can get groups together.)