Well, the year is a big part in these bikes. Sense you said the Zx-6RR you must be looking at the 05-06 or 04-03. The RR is the model Kawasaki had to release to be able to run in the 600 cc races. Because the Kawasaki Zx-6R used a 636 engine.
IF this is the bike he is looking at I would recommend the Zx-6R not the RR. The 636 engine is legendary. The newer 6R ninja's have been getting allot of grief for not having the 636 anymore "07-09".
The 06 Kawasaki Zx-6R is an incredible bike.
The Yamaha Yzf-R6 probably isn’t a bike he would want if you are looking at the 06-07 and 08-09 R6 with the new styling. The R6S however isn’t as bad.
The R6 models are uncomfortable, they feel cheap, and the engine is a joke. The bike has virtually no Torque in favor of a couple extra hp up top.
For street riding Torque is something very valuable with these 600's.
Hope this helps