I have never used RTB Hosting, so i can't realyl compare the two. However I can tell you why I chose Kaphost.
I really just needed a simple server in a better location. That's what I was shopping for. So when I saw this:
and now we're proud to be able to solve the issue of being able to host a quality Blockland server regardless of your physical location or technical knowledge.
I knew I'd have to go with Kaphost. I'm a tekkie. So when I saw that RTB was designed for the more.... simple users I decided I really didn't want to struggle with that. After checking to see if Kalphiter could be trusted with my money(he seemed very professional with his clients on the forums) I decided to go for Kaphost.
A month or two after I started using Kaphost, if I remember right, the service stopped accepting new members, and all existing members kept their servers for free, until he finishes the new panel.
So, my advice to you would be to use RTB Hosting for now, but to just try out Kaphost when it comes out.