It depends on what your disposal cost are and how far your dump site is. A chipper makes life a lot easier but for small loads I dont bother. I only rent a chipper when I have 20+ yards of brush. Otherwise it is cheaper to just load it and haul it. If you only have a few pickup loads I cant see a chipper being worth it.
That being said, if you are set on renting a chipper, I would rent a 6" hydraulic feed unit. All of the small units are just a pain because the branches have to be chopped up small and must be straight to get them to feed. I did use a bearcat 5" chipper once which had a large infeed so I didnt mind it. If your rental place has a 4-5" with a large infeed you might be ok but I would still get a 6". I usually do all the cutting first and line the brush up to be chipped. Then I go get the chipper, chip, and return. This way I only need it for less than 4 hours which is the minimum rental.