It literally depends on the budget you have and how many total units you intend to make - but if low budget is key to the equation, you start off with just one and show it around while making it the best it can possibly be, like Scott Danesi did.

All IPs start off as 'original' themes. The point is they become franchises for a reason, when the ideas, story and marketing people have all properly done their job. You should be thinking about a pinball machine the same way, and If your budget isn't so limited, asking yourself the relevant question - does your team possess the talent and necessary time to develop a fresh idea into something inspiring, or do you pay to have that work already done for you provided you know exactly how you want to build it up. There should be a 'both' option in the poll.

The fallacy in this question is the assumption that the guy holding the purse strings hasn't already made the choice. :/