On topic, I prefer Nf6. First, because I can play something else than ..d5 systems against colle, london and these systems. Secondly, because I prefer the sharp games arising from the Modern Benoni:
Which do you prefer against 1.d4 and why, 1...Nf6 or 1...d5?

I'll have to try that next time I encounter the London system.
In regards to playing the Modern Benoni, what if white doesn't play c4 on their second move? It seems like black has many interesting options with 1.d4 Nf6 but most of them depend on 2.c4.
Also what about playing the Czech Benoni, how does it compare to the Modern Benoni in your experience?

On topic, I prefer Nf6. First, because I can play something else than ..d5 systems against colle, london and these systems. Secondly, because I prefer the sharp games arising from the Modern Benoni:
As a former London-System-Player (i switched to the BDG long time ago) i was very pleased, when my opponent played this d6-stuff, because i think its not the best reply to the London,f.e.
And White has a slight advantage...

FriendlySquid: Yes, 1...d5 positions will be significantly different for you than the King's Indian you have been playing. 1...d5 is solid at the cost of some dynamic possibilities inherent in Indian systems.
Be aware that the place where 1...Nf6 and 1...d5 intersect is the Catalan.
1 d4 Nf6 2 c4 e6 3 g3 d5 4 Nf3
1 d4 d5 2 c4 e6 3 Nf3 Nf6 4 g3
They often lead to very similar if not identical positions.
I think the reason 1...Nf6 is most common is beacuse, as someone already pointed out, it's the most flexible, as the black knight will go to f6 in almost all variations.
Nf6 retains more options. You can play the King's Indian Defence, Nimzo-Indian, Slav Defence, Queen's Gambit Declined, Semi-Slav, etc.
Of the two most common moves against 1.d4, 1...Nf6 and 1...d5, which do you prefer and why?
Do these two responses lead to characteristically different positions? I would especially like to hear from people who have used both and how they think they compare.
Right now I play 1...Nf6 and while I do ok with it, most of the games don't seem particularly fun to me. For example, The London System, 1.d4 Nf6 2.Nf3 g6 3.Bf4 A lot of times my white opponent will do this. Other times I get stuck playing a King's Indian position in which my opponent is using their pet line they've been playing for years, meanwhile I've only given a cursory look at King's Indian Defense theory because I'm busy learning the Sicilian as black and the 1.e4 openings for when I play white.