5 dollar on 500 coins or 15 dollar on those coins and a month sub. I personally took the second approach, seemed better (especially seeing I haven't even used any of those 500 yet).
But, your choice.
After a sub ends you're automatically a preferred player, even if you haven't bought cartels coins before that, or after that.
If you buy the game all you have to do is enter the keycode to the site, and 30 days are added. No cred requirements, no payments afterwards needed. After those days are up, so is your sub status... you can re-activate it whenever you want though if you miss it. Personally I don't play nearly enough to warrant $15 dollar a month (13 euro a month). The pref. status with unlocks suits me (and my gaming style) a lot better...
Now, what's the best to spend cartel coins on... Rocket boots?