You have read the spec sheets haven't you? I believe it is still accessible at
http://www.akaipro.com/contentmgr/showdetails.php/id/2644 if you are so inclined to read up on it. You state you own a 1k already and have used the 5k.
The 1k with JJOS has a fairly comparable feature set to the 5k. The 1k is more portable than the 5k. The 1k has been out longer and has more docs and users behind it.
The 5k has more output options. The 5k has more input options. The 5k has more memory. The 5k can be upgraded to larger hard drive storage. The 5k has the option for a cd/dvd drive. The 5k has an intergrated synth.
What do you want to know? Which one is better? I don't think anybody on the 1k or 5k forums are going to tell you which one you should own. There is probably some merit in owning both for yourself so you can decide. You've already got the experience with the 1k and the interest in the 5k. If you find a good deal on the 5k the resale of it if you hate it probably won't cripple you financially, and the resale of your 1k you're going to take a hit anyways.
Good luck man... let us know what *you* think.