There are those who swear by all off them, oil, water, diamonds, sandpaper, ceramic. If you are into power sharpening, stiff felt (best for round bevel knives) leather wheel, mdf with compound or the paper wheel kits. Ultimately power sharpening will give the sharpest edge and quickly considering the edge is there and just needs honing. Bad part is there is little margin of error, not portable and can be dangerous. Also quick way to ruin the temper of a tool.
If I were to have to pick one way, I would get a double sided diamond "stone" of 600/1200. Unless your using it every day it will outlive you. George Strunk turned me on to E-Z Lap brand and I've been pretty happy with them.