We had decent response from boatus on ICW.
A quick comparison (looking on their web sites) shows that BoatUS has same number of locations as SeaTow in North Carolina and twice as many in both South Carolina and Georgia. I did not check other states, but it is easy enough to do.
And of course BoatUS has a bunch of other useful services and insurance to boot.
It's really hard the say, but generally I would guess that in that area they are equally distributed. I am in the Nothern Chesapeake Bay and in my marina a Seatow boat is docked. In the marina next door is a BoatUS boat. I went with BoatUS because it was a little cheaper and you also get the benefits of membership.
BUS was my choice and never had cause to regret it. If it were a local deal...use whoever seems to have the better coverage in the area...but for the ICW...I don't think it much matters as both are always near enough. I give BUS my business cause it is a good org and they've treated me well on the insurance side and membership benefits over the years as well.
I am a very big fan of Boat US, just do not make the mistake like a lot of sailors do, get the unlimited owing, it covers you up to 25 miles from any of thier bases.