With new versions of Alternity funded on Kickstarter, Traveller just released, and the StarFinder announced, which SciFi/Sci Fantasy game would you suggest investing in?
What is the difference between them?
Thanks for your help!
Which depends upon what you want. They're not the same genres/subgenres.
Traveller is hardish space opera to moderate space sci-fi. Yes, it's got Artificial Gravity, FTL travel (but not comms nor sensors), and a system design esthetic that was (but isn't now) one of the most realistic before the Kepler 'Scope. Most PC's are ex-military or ex-merchant, and the mechanics (in any edition) are reasonably traditional. My recommend should you choose it is either CT, MT, or TNE, depending upon your preferences; I don't endorse MGT, but it's workable (feels wrong in play, like a bad retroclone attempt).
Alternity is pretty much middle-of-the-road space opera, at least in the old Star Drive setting. Suffers from the usual issues of dice-by-difficulty (hard to not reveal the difficulty, 1E is roll low), and what I've seen doesn't look like they're trying to muscle in on Traveller, but to go in a different direction. I never got into the Dark Matter setting, but it looks pretty much Spacefaring Cyberpunk. Some of the materials under Retail Play were in the border overlap of Space Opera and Space Fantasy.
Starfinder is going to have the issues of Pathfinder in Space as well as being firmly space fantasy. I don't care for pathfinder mechanics, and I get all the Space Fantasy I need from either Star Wars, Space 1889 or Spelljammer. So, I've been keeping only a vague eye upon it.
There are many more choices.
My personal recommends would be CT if you like OSR (buy the CD), TNE if you want a feel closer to Gene Roddenberry's Andromeda, or FFG Star Wars for, well, star wars or the border of Space Fantasy and Space Opera.
Edit to add: I'll also recommend the Fading Suns game, at least for the setting (rules-wise, it's easily able to be swapped out). Kind of 40K gets a fresh infusion of Dune.