I've played Freelancer and Darkstar One (I've also had a taste of the classic Escape Velocity back in the day). I'm familiar with the style, but what makes me excited about X3 is it seems like most open (and best looking) of these games.
I like the space games, and I imagine that EVE is the MMORPG of that category.
Oh yeah, X3 is gorgeous. There's no way around that. It's very, very purrrrrrrrrdy.
So, if you do like Freelancer -- and me, I did love Freelancer myself and consider it like the GTA of space-games -- you'll love X3 even more so. X3's a better game, no doubt.
And yes, X3's definitely more open-ended than Freelancer; no real comparison there, since X3 doesn't basically even force you to do the main quest. Unlike X3, Freelancer tries to keep a balance of things, since it does try to keep a good cinematic story going -- you will do some side quests first for Freelancer, then a main quest mission opens; then you can do the main quest or do more sides. X3 is more like -- "Okay, just go do what the hell you want; follow the main quest, if you want; do some sides, if you want; Start your own trade route and clan of ships to join you; whatever, you're the boss..."
You won't regret X3, bro...