Old/Modern Benoni vs Dutch against d4?

Which one's better and why?

If I had to choose between these it'd be the dutch because there's much more complexity to it, which brings with it opportunity. If you know the lines and your opponent doesn't... which he probably won't know them, if you play very precisely you can gradually get an advantage. It's still a difficult game because you weakened the f pawn, and one wrong move the whole position can explode on you. But in contrast the benoni is just easy to play against for white, white really can't go wrong, even if your opponent isn't prepared he can follow basically the same pattern of moves and you're just going to sit back, have a bad position and slowly inch your way toward getting your pieces out.

Now the benko is an entirely different story... it's a great opening. Anyone who wants to play the old Benoni I'd just tell them to forget it and play the benko.

I would chose the Benoni.
https://www.chessgames.com/perl/chessgame?gid=1044749

Definitely the Dutch.

The old benoni is dubious because White has a big advantage NOT playing c4 and leaving that for his Knight. 1.d4 c5? 2.d5 d6 3.e4! Nf6 4.Nc3! And White will play in some order the kingside pieces, castle, and Nf3-d2-c4, and presumably Bf4 at some point, going after that dead duck on d6 if Black ever plays ...e6, and if he doesn't, enjoy the massive space advantage and don't trade pieces.

Modern Benoni the problem is once again the dead duck on d6.

The e6-Dutch lines (not the Leningrad) are far sounder than the Benoni.

Both of those openings are perfectly fine (even Old Benoni, at a certain level), and asking which one is better is bssically like asking which color is prettier. Choose an opening which suits your style better. That can be done by watching couple (the more the better) of games played in those openings, and seeing their typical middlegame plans. The one you feel better is the one to study further.