Luckily going forward it'll be pretty easy in the stem dept. Most decent road and MTBs have 31.8mm clamp diameter these days. Both realms have also adopted the same diameter for the next size up - 35mm.

The 31.8mm agreement is one of the best things to happen in cycling in the last 20 years. That and Campy/Shimano/ Sram cassette spacing being the same for 11 speed. I still can't believe that one but I've heard it through the grapevine.