Both offer similar performance. It all boils down to your personal preference. If you are a gamer or consume a lot of media content, you need a bigger screen & so the zenfone 6 will be the one you go with. Remember though that it is a humongous device that is taller & thicker than the galaxy mega 6.3.
If you want a pocketable phone, the moto G is the one. Being an older & more popular device, you will always find adequate support when it comes to software updates, custom roms & kernels. If you are a heavy modder, I suggest you go with the moto G.
Finally, take out some time & read the reviews of both devices. I suggest that you also visit sites like amazon & ebay & read the reviews of people who have actually purchased & used these devices.
Do your homework & decide. All the best.