I think the rocker switch is a better choice. Easy to operate with gloves on. Toggle switches are too easy to clobber in the heat of battle.
If you don't understand electricity, then you look at all of this as on/off.
It's anything but. When you flip the light switch in the house the light bulb is cold and looks like a dead short to the switch. This is called inrush current. A 60 watt bulb that normally draws about 1/2 Amp steady state current, the inrush current is about 7.5A. Have you noticed the difference in size between your dashboard switch and a wall switch?
Motors have a coil of wire that acts like a balloon. You not only have starting current to get it rolling, but all that energy collapses through the coils as the switch is opening.
There is no difference between a switch and a relay electrically. You could put a huge switch in the dashboard, but that rarely works out space-wise. It's easier to put a small switch in the dash that operates a larger relay.
Switch or relay, there are contacts that look like this:
The large arc happens when you open the switch under load. A smaller arc happens on closing the switch and again when it bounces. If you have seen pitted contacts on a points and condenser ignition, now you know why.
Why not put a much larger contact in the switch? Contacts oxidize over time and the arc breaks through to make a contact. Wetting current is the term for sizing a set of contacts so that enough current exists to break through the oxide. Too small a current for the contact size can result in not breaking through the oxide and the switch doesn't work.
Dang engineers are required for everything.
Here is a relay and a switch primer:
http://www.phidgets.com/docs/Mechanical_Relay_Primerhttp://en.wikipedia.org/wiki/Switch
