I can't say much about RPM as I've never used them but I just bought the MPIV2 from PAW for $299, 2 credits cost another $99 and then PAW will tune for $375. you only need a PC next to your car to install the tune, but you'll need a laptop (mines a cheap $199 laptop) to do datalogs while driving (or a PC with a big UPS, lol). The HPtuner Editor is what you use to add/remove a tune. The HPtuner VCM scanner is how you datalog. Both the editor and scanner are free downloads from HPtuners website. Dave from PAW wrote a base tune for me and then has revised it several times, after some data logs. The $375 from PAW gets you unlimited tune support from Dave (he'll make as many adjustments as needed for free).
I went with PAW because they seem to have a stellar tuning record, unlike RPM.