Well, when the MH come on it takes a couple of minutes for them to go full on. It is not a super fast transition from completely dark to very bright. However, when they go off, there is a only slight lingering glow while the bulbs cool down but the change is pretty drastic, at least on mine they are. That is why I would highly recommend the actinics as well (Greg can give you are good deal on the T-5 setup as well) and put them on at the same time. That way you don't freak out the fish when the lights come on or go off and they will be less likely to jump out of the tank.

Just a thought.