Russ,
The one small chipper I have seen in use was the Vermeer 625. It actually works pretty good, for its intended purpose, The smaller stuff.
They are not superfast, but their auto feed seems to work well and the can be put into back yards pretty easy.
I think the Promarks always had superior infeed speeds though so if comparing it to a Vermeer unit, I'd take the Promark. Promark is about the same as an Ariens/Gravely chipper.
The reason that I`m looking at a small chipper that I can get into back yards with, is because I think I can sell alot of pruning and clean up jobs in a nearby city which has many houses backed by steep hils and ravines, and subdivisions backed by "green areas" that the chips could be blown in to. This would mean less dragging/more productivity of the laborers. Hauling and dumping chips is not a problem locally but it is an added expense that is often unnecessary.
If you need a small chipper like that, you can't go wrong with the Promark 310. Actually, all of the Promark chippers are great machines. Gravely bought them out, made chippers for a while and then sold the chipper manufacturing rights to someone else.