I hear the opposite where Unity is quite stable while GNOME crashes more often. That's from using them as normal desktops which you probably aren't going to do with a mining rig anyways. As for resource usage, I don't think that's a big concern since most mining rigs have 4 or 8GB of RAM and running Unity + miner uses less than 2GB. I don't think switching to Arch is a good idea. Closed source miners are compiled for Ubuntu and running them in Arch will result in miss and/or wrong shared libraries.