Pull-starts are easy and convenient but they also put more resistance on the shaft which will slow your car down a bit, where as starter boxes put no strain on the shaft allowing for the engines full potential (that is why most racers use boxes) and i suppose if your really picky; having no pull-start will be less weight, and with a starter box you have to charge it up ext, which shouldn't be a problem unless you came to nitro because you didn't like charging battery's (just stick it on charge with your igniter) and also pull-starts snap. If you are just bashing the pull is probably the way to go, if you are thinking of racing competitively where you are squeezing as much speed and acceleration from your engine go with the starter.
I use a pull if that helps as i cant afford a starter.

Tom.