[i have crabs]

I think your options will depend on your plans for the tank.
I assume you mean that the tank will be viewable from both sides and 1 end pannels?
If so having powerheads on the viewable end will kinda suck so i think vortechs on the nonviewable end would be nicer.
Also if both side pannels are viewable then the rocks will probably be stacked dpwn the center of the tank so 1 vortec wont be that great, 1 vortech blowing down each side of the rockwork would be much better, mp20's shpuld be plenty in a 36" tank but if the money isnt a factor i would get mp40's and dial them back a bit if needed rather than find out you dont have enough power.