Which set of endgames is it harder to study;  are Rook endings harder to study or more difficult to carry out or are Bishop endings more harder to study and/or difficult to carry out the win ?

Bishop endgames i find very tough

Though question. 

I personally really like rook-endgames so my personal opinion is that "opposite colour" bishop-endings are harder to evaluate especially the line between a draw and a win. Also it can be hard to convert an extra pawn in a same colour B' endgame

Then again I'm waiting for someone with a different opinion.

Rook endings are more difficult to win than same coloured bishops, they are often very drawish.

I think rook endgames are the most difficult and I have read so in a lot of books, but of course bishop endgames aren´t easy. 

I don't think there is a simple answer: everything depends on the position. Rooks and pawns are an interesting combination, because the pawns are unidirectional (forward) but they have that interesting diagonal capture movement. Add to that the possibility of en passant captures, and R+Ps can be a real challenge. I, personnaly, have lost several drawn endgames because I missed threats my opponent had.