Do you ever want to stack bags? Duffels don't support (literally) that function, so a sturdier option is preferable. A duffel can smoosh down, which some like and others hate. My husband has a rolling duffel (north face) he LOVES as a carry-on, but hates what it looks like when it's not full, so he'll stuff fleece jackets etc in there to fill it out. He can stack that bag on a larger rolling 2 wheel piece of luggage that he checks, for walking in/out of the airport and to the hotel. Alternatively, some duffels might have backpack straps for a different sort of flexibility - you could roll or carry the bag on your back.

Personally, I don't like rolling duffels and prefer the feel of softside rolling luggage. 2 wheels :-) And sometimes it comes down to that personal preference...