I'd also assume wool.
There's some non-canon work out there somewhere that postulates that the reason God was displeased with Cain is because Cain was complaining with Abel's sheep eating from his garden, while Cain didn't recognize his own obvious wearing of Abel's wool. Hypocrisy I suppose would be the fault there.
I prefer to think that Abel was more pleasing because he was a caregiver and Cain was obsessed with thinking.
Shepherd vs. tiller of the ground.
I guess both those theories are kinda the same.
As to the question why was Abel a shepherd, maybe he just liked it.
btw, nice thread title lol