Hello there.I prefer pokerstars ....there are more players more games and there is a lot of value at low stack mtt.For playing omaha is the best poker room in my opinion.Good luck at your games.
It depends on what your buy-in range is but I would consider using both. If you are able to get enough games on one site then you could pick one over the other. Though from what I've been told by friends who play on them both, party poker is a lot easier to see consistent bank roll growth.