Really depends on how central this issue is to the gameplay and how quickly you can get the two options up and running.
Big Central Issue
If you are talking something central, like half the team wants to make an FPS and the other half an RTS then there are no easy solutions. Look at the space you are going to be releasing the game in (web/phone/...) and go with the one that's more commercially viable if you plan on selling it.
Easy to prototype issue
If the problem is small and can be easily prototyped you can at least try both versions out. Unfortunately if there isn't a very clear winner in the prototyping it will suffer from confirmation bias and both sides will still think their side is right.
Avoiding in the future
You need to get out of the design by committee mindset, as mentioned by AttackingHobo, it's not conducive to making quick or coherent decisions. Unfortunately if no one of the team has experience making games it can be difficult to get everyone to follow one person since everyone thinks they can be a designer.
While you are learning the game development process I would suggest looking at other games similar to what you want to make for a lot of solutions. While you don't want to just make a carbon copy of an existing title you only need 1 solid new feature to differentiate your game. Outside of that core feature just copy what the market leader for your genre is doing. You'll learn a lot about what they are doing right and it's easy to justify a design decision by pointing at a successful title and saying we are doing it just like that.