Decrease FPS of too-high FPS video or increase FPS of too-low FPS video?
I think the answer depends upon what frame rate you want to deliver your video at. 

In general it is better to shoot at the delivery frame rate. 

I agree they probably won't go far out of sync if the clips are only short (a minute or so). 

If you do need to adjust them, I would frame rate adjust whichever clip is going to be on screen for less time in the edit (or is less important to you).