Both have their own merits, I will probably say Texas Holdem has more players due to televised events and being called The Cadillac of Poker by Doyle Brunson it has always been the most popular form of Poker, but Omaha is well worth learning as some players register by accident and don't realise until the game starts, by then of course it's too late to leave and get their registration back.
I have lost count of the number of players who have been dealt 4 of a kind in their hole cards and pushed the betting up simply because they think they are holding the nuts, and do not realise only 2 cards count, or on other occasions make an Ace flush with just the bare Ace thinking it is enough to win when in fact they need 2 suited cards in the hole.
Omaha is by far, a more complicated form of Poker and well worth checking out, but I would advise using play money first to get the hang of it.
When the fun Stops, STOP!