Soda fountain offers the lower cost, but, has a lot of side issues associated with it. Either you will need to but a lot of ice, and hold it on the truck, or you will need an ice machine, which is both an electricity hog and they often break down. The idea of using the cups to advertise is nice, certainly what 7-11 and fast food places do, but, you might want to measure whether you want your product associated with those places, which marketing as they do, will most certainly be the result. Further, you are adding to garbage logistics, where as cans can be recycled.
Bottles and cans, especially bottles are viewed as an upgrade, plus, you will have more options available to change or expand your menu. They are discreet packaging, that is to say, you do not have much in terms of labor or serving to get them off the truck. It will be just two steps you don't have to do. You can reload beverages at any location, without the hassle of keeping syrup or CO2 bottles.
Either way, you have an ice issue, if you plan to use cups. If you go straight bottles and cans, you can use refrigerator space or a dedicated ice cooler. Load up once or twice a day and you are done with ice.
The SS Platypus, sails again, to the support of the Vets!
"perhaps...but then again...maybe not..."