Have "neither" as an option is what I think
Volt-Tackle is good as a kamikaze move, ie - you have 5 hp left, you know you are faster than the opponent, so if you die, might as well bring them down.
But thats only good sometimes. Go with thunderbolt > thunder all of the time, unless you have a rain dance set (you have no business doing that as Surfchu is better with sub > rain dance anyway).
My point - get the lightball, not volt-tackle.