This is just an opinion. NLH is easy and for everyone and a high percentage of players are quite profficient at it, but it's also for gamblers, at least in microstakes. PLO requires slightly more skill as it is a more complicated game with many more variables and possibilities, so it is more difficult, but there are still a few players who can play it skillfully, so there is still decent competition even though far less than in hold'em. PLO5 however is a newer game that requires a lot more study, a lot more skill in hand picking and a lot more gambling balls, higher variance, but the field of players is below average.
NLH you can multi-table, PLO is more difficult to multi-table. PLO5 is pure insanity to multi-table, I can do 3 tables at most without going mad within one hour.
I prefer PLO5 because I feel like I have more of an edge and have been trying to focus most of my time on studying and practicing it, particularly because of the headless gamblers involved in it. It can be boring, it's mostly folding which can be frustrating, but it's a great opportunity. I still need to put a lot more effort into it before I can say I'm good at it. Other
poker games are rarely offered or played, other than
pokerstars and swcpoker, so it's pretty pointless to consider them for online, as you rarely end up playing them and therefore a waste of time jobwise.
Try, pick, focus and get good at what you like. At least, there are options.