I have the RB K9, I don't know how much different the K10 is. It's fast and gets good run time, but it's no Novarossi. It doesn't hold a tune near as well and doesn't get as good of run time compared to the +4.

The P5 is probably a little too much motor for a buggy, I'd go with the +4 over it. It's a smoother motor and gets better run times. I run the +4 in my truggy and the K9 in my buggy right now. The K9 will soon be replaced with a Nova though.

I've heard great things about the Werks motors, but have never owned one so I can't comment.