If the KG5.2 are in good shape get them. I owned a pair of 5.5's years ago, and they were pretty darn good. Replace the tweeter's with Bob Crites Titanium Diaphragms, if they are still being sold.
I would stay away from the RF600B, from what I read seems they are lacking in the mids. Rumor has it they are just rebadged RF62's version 1, speakers that are sold exclusively for big box giant Future Shop in Canada.
You best bet would be to listen to them both, to decide which sounds good to you.