We are going on our 4th year and I couldn't agree more with JConk. We have an open trailer with an exposed oven and the visual aspect is huge for us. We do a 3hr. farmers market and for the whole three hours I have people checking out the oven, from daddy's bringing their kids to watch their pizza bake, to elderly customers chatting about how they would heat their meals on the engine of a Sherman tank in WWII, to the scientist/engineer who wants to talk thermal dynamics. We also do several local wineries and private parties and at these events I would honestly say that half of the guests check out the oven and strike up conversation. I really believe the open oven helps boost our sales and increase the word of mouth events we get called for.
With that being said I think your vision should drive what your set up is like. My vision was exactly what we have happening, a great amount of customer interaction. If your vision is to show up outside a business and crank out some lunch time pies a truck may be a better set up for you, quick and easy in and out. And like JConk, just my opinion.