For the relative cost, I suggest you join BoatUS and obtain the Unlimited Towing Option which is relatively inexpensive. TowBoat US has many stations in the Keys and in the unlikely event they cannot get to you, the policy will cover towing services by a private contractor (Sea Tow). Moreover, with the BoatUS Membership you can obtain discounts on transient slip fees and fuel at many locations in the Keys which can be quite substantial given the costs for services in that area. Sea Tow is a good provider but, in this case, the advantage goes to BoatUS.
Boat US. There is plenty of coverage in the keys with Boat US. SeaTow has all but disappeared here. The only yellow(SeaTow) boat I ever saw in the keys has painted out the SeaTow markings and no longer works with them. Between Marathon and Key West, there are at least three bases to help you with Boat US.