Mitch,
Honestly, the RIP doesn't make just a whole lot of difference with the 300 series latex printers mainly because so many of the inking controls that are possible in a RIP on other printers have been taken away and the machine has been made into a very restrictive contone-only printer.
Also the entire series has some serious color inconsistency issues that are caused by the way it handles how it manages its onboard "calibration" routine -- which I have confirmed through my own testing -- and also -- so others have reported but I've never actually confirmed -- by the configuration of its light cyan and light magenta printheads. As would be expected, these issues are most notable the closer you get to neutral, so if you're particular about consistent color and/or you print a lot of greys, you might want to reconsider your selection of printer.
Aside from that though...
I wouldn't advise Wasatch. Not unless cost is your main concern. It has no built-in Pantone library and building spot color libraries in it is possible but extremely annoying. It's easy to learn and use, but its main selling point is price.
Flexi... Meh... Not my first choice, but not a disaster.
Lots of people like Fiery XF and with good reason. I'm not really one of them, but it is a front line RIP.
Me, I sell Onyx and I sell Caldera, so I'm kind of partial to both of them. Both of them are full-featured front line RIP's, with lots of printer and inking control features the 300 series won't let you use, and both of them have their strengths and weaknesses. Some people love Caldera and hate Onyx. Some people the other way around.
Some people will tell you that Caldera is blazing fast, and it usually is... but my experience has been that's mainly due to the fact that it's always on a Mac or a dedicated Linux box, and never on some wheezing old bloatware-infested Windows machine. On comparable machines, they run about the same, which is fast enough that RIP time is for all intents and purposes never an issue.
My bottom line would be that if I was you -- honestly -- I'd pick another printer first, but then, if you tend to think of yourself as a Windows guy, you'll probably like Onyx better. If you tend to think of yourself as a Mac guy, you're liable to prefer Caldera.
Mike Adams
Correct Color