There are a number of layers to this one, but if we put aside specific expertise, some brief notes -



I'd look at the opportunities, and try to figure out how the market will look in the 12-18 months it will take to finish the game.



At the moment it's fair to assume that PC will remain a strong and stable platform (tech but also retail stores) and a potentially big audience. It's also one of the more straightforward platforms, because it's easier/cheaper to update. So it's a fairly safe choice - the major obstacle is visibility, since there are so many games being developed for PC.



For consoles, both PlayStation and Xbox take indies seriously, and actively look to support projects with access to their platforms, event hosting, etc. Both have a good audiences, and although there are different approvals processes, if you've got a strong game, you've got a good chance at visibility/support.



It's great to see Nintendo welcoming indies as well now - Switch could become a viable addition to the above, which would be lovely.



iOS/Android is very tough, just because of the sheer amount of competition, and the process of acquiring users.



Vita... personally I think this is a relatively untapped market, and could be a good opportunity, though I'd combine it with other platforms to avoid putting all eggs in one basket.



Generally we see devs making games for PC, with plans to port to console (either at launch, or later). It's a sensible route.