Hmm, you should consider what you really need. even though the 4 or 6 ball rollers are handy because you have "everything in one bag" and can take them apart to get several totes, I found that they are very clumsy to haul around. They are very broad, and must bowling lanes I know are not too spacious... Additionally, if you are to haul the bulk into a car trunk, these will take some space, too.
A friend of mine just built a multi-ball roller from a foldable cart and some Brunswick 2 ball totes, all the material came from ebay for cheap.
Personally, I found that a 3 ball roller plus an attachment tote for the handle or a 2 ball tote are the optimum for my personal needs (and availabkle car space).
Another thing I found generally good are big and braod wheels. Make sure that it has at least 4" wheels, and urethane "tires" are nice, too. A thick hanlde will also make dragging the bag easier, especially when it is heavy and you must make long ways.
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