@ the OP:
"...So is it better to be good at openings or endgames?...Any suggestions?"
Currently my favorite reading is "Zoom 001 Zero Hour for Operative Chess Opening Models" by Bent Larsen - a generic system that tries to cover all aspects/phases of the game - opening, middlegame and endgame.
In short: You play trough about 400 games with "Grünfeld glasses". The book includes/is about the handling of Queen's pawn, Dutch, Reti, King's Indian, King's pawn, Catalan and English structures for white and black.
"You may very often find yourself involved in an endgame - with or without queens, mostly a rook ending. And mostly the endgame is reached very quickly - which is the logical result of the breakdown of the center and the subsequent fight for the open files. So - if you don't like endgames then don't apply Zoom 001 - and forget about playing chess." (page 14, ibidem)
Please see
http://www.chessgames.com/perl/chesscollection?cid=1018065 for details.
Regards, Karl