My money is on Colorburst for a few reasons.
First, it's an integrated product with Epson, so, in my experience, you get support from both Epson and Colorburst. Colorburst runs a great online forum-style help, and they're on it. You often get answers immediately.
Second, it simply works great. The newer versions are much improved, give you a lot of control, but also allow you to run it out of the box very well.
Ideally a RIP is an appliance- you want it to run like a toaster, so the root of it is reliability and support, whether you're using it for proofing, fine printing or just ripping PS files. Ironically, many RIPs lack just that, websites are bad, no support, no customer service, whereas ColorBurst has always been right there. In theory, too, you'd expect because of their relationship with Epson they're going to continue that effort, although that's not how EFI went with HP, IMO.
As far as quality of the print goes, I'll just say I've seen no discernible difference between prints made by both, but it was hardly a scientific comparison.