vessel assist not a major player around here, so just can't comment.
i recommend you monitor VHF for a couple of weekends, see which outfit gets more biz, responds quciker, fewer cusswords from irate policyholders, etc.
i've never had to use any towing service. but do have sea tow, and might switch to towboat us for renewal.
salvage policy: towboat us, hands down (read both their contracts and you'll agree -- plus, do a search on this board about salvage issues w/ sea tow). but, you can avoid most problems by asking outright BEFORE services are rendered whether it will be a tow or salvage operation (soft vs. hard grounding, "imminent danger," etc).
towing distance: sea tow wins. not issue w/ me, however, as i'm a boating home boy.
dispute resolution: boat us seems to be much more consumer-friendly, per their contract.
coverage issues (boat v. person is covered, etc.): for reasons mentioned, sea tow seems to win (but i THINK you can upgrade level of service w/ boat us to match that of sea tow's).
number of boats/response time: seems a tie down here, maybe slight sea tow slight edge.
misc: sea tow. my kids like the yellow boats better than the red ones