You can find a number of comparison charts/posts like the above provided (the reddit chart is good too), but all of them are incomplete (not that my additions are either) because there are just so many changes. There's also the ability to make characters of certain species for certain classes that subscribers get, but preferred do not (unless you've unlocked the species in your legacy). Subscribers buy from vendors for 80% of the credit cost that preferred do, gain increased experience, get rest experience, have the ability to exit stronghold direct to ship and direct to fleet (greyed out for preferred), and get additional requisition in GSF. Basically, if you can think of a way the game will make your life more difficult to not have a sub, they do it. I was surprised how many undocumented (or at least rarely discussed) restrictions are removed (or benefits applied) there are by subscribing and I'm quite sure I'll keep finding more. If you do any manner of selling on the GTN, the 5 slot limit per character as preferred is extremely restrictive, so I'd pick up the unlocks for more slots.