I would echo what others have said here. I would not pick a path based on price, as they are both very similarly priced when you know how to compare them correctly. Also, I would not pick a path based on digging into the technical requirements and trying to say "oh this one supports this but not this..." I say that because projects like these evolve over time. What is important to you when you have your wallet out might not be important to you at all in 6 months. You may envision the entire process differently in a way that makes whatever that one deciding factor was irrelevant.
I really think it's about community, as others have said. I would encourage you to join up at the pinballcontrollers.com forums and get active. Introduce yourself, ask questions, chew the fat, etc. There's no pressure to buy a P-ROC there, and really if one wanted to they could code their entire game before they bought any hardware at all. On the pinballcontrollers.com forum you'll see many such videos and screen grabs of projects running virtually, and there are a few different technologies you can use to accomplish this. VP is one (which I used to use), but OSC is the one I use now.
The guys over at pinballcontrollers.com are top notch and always very responsive and willing to help. I've gotten help from a number of them at various points along my journey. When i think about the hours guys like Gerry and Michael (MOcean) and Mark (Snux) and Jim (Applejuice) and Scott (NoTrashCougar) and others have carved out to help me and others like me... Well, I'm speechless by their generosity.
Okay, unrelated to Fast versus P-ROC, but curious to ask... I'm re-writing the rules to Pinbot, and while I'm making great progress and very proud of my work, I admit it's a LOT of work. Designing and building an entire game from scratch and THEN writing the rules? Yikes... That's a big project. Do you have a buddy, like a childhood friend (or three) who also loves pinball and who shares your vision? In my case nobody was interested, but I admit I'm always jealous of those projects where a couple guys who grew up together join up to knock out a game over a fridge (or three) full of beer. If not don't fret (there are many successful one-man shows in the P-ROC forums), but worth asking, IMO.
Anyway, good luck. Whichever technology you choose, I'll look forward to seeing your masterpiece.
Dan