[i have crabs]

they definatly provide the widest flow pattern but the only way its gonna cove both the viewable sides is if its placed in the center, but having the tank viewable from both sides implies that the rock will be stacked down the center of the tank. is that your plans? if so having 1 vortec in the center is just gonna blow strait into the rocks and not really provide flow to the whole tank,
in my opinion even a mp40 isnt gonna provide overly strong flow after 36" of tank
i really think your decision is gonna depend on your layout of the tank and the budget you have to spend.