The first thing I absolutely MUST say about this game is that it's a LOT of fun! It great. And while I've played both games (Star Wars and Star Trek), this one has held my attention since I started it. The most important thing I noticed about this game is that, while Star Wars: The Old Republic starts you out in a very small section of the world map, practically discouraging any kind of interaction with other players, Star Trek Online sends you RIGHT into the action. Exploration isn't only encouraged, it's practically mandatory.
Exploring the universe is part of the Star Trek universe, and it would be absolutely pointless to put together a game that didn't make exploration integral. It's very easy to group with other people for a mission, play on your own, dedicate 20 minutes or 20 hours. You can explore or fight. You can put together diplomatic missions, science missions, rescue missions, and anything else you may be able to come up with. AND the geniuses at Cryptic put in a mission designing program!
You also get a lot more out of this game in the free-to-play mode. There are bonuses to paying for a subscription, but you're not a "second class" player if you don't spend any real-world money.
I want to delve into both games more because they're both fun. But for now, Star Trek has SERIOUSLY grabbed my attention. It helps that I've been watching the shows on Netflix.