I think many people will recommend that a reel is preferable to a spool for towing a float, A reel allows the diver to more easily adjust the length of the line to address the specific environmental conditions.
For example, if the current is strong, then it is advantageous to have a lot of line (scope) out because it helps reduce drag. If there is no current, then it is often beneficial to keep the tether line short and close to vertical so as to avoid the opportunities for getting yourself or a buddy tangled in the line.
Also, if you are drifting along a shipwreck that has towers or parts sticking up vertically, then if you have a lot of line out, you can get tangled wth structure. It is often better to shorten the line as much as is possible to reduce the chances of getting the line wrapped around portions of the wreck.
Also a reel allows the diver to shorten the tether on ascent or with varying depths. It is often nice to reel the line in on ascent so the diver can do a safety stop directly under the float and then ascend immediately adjacent to it as well.
Some divers will select rope rather than a reel and thin nylon line. If you select rope, it is probably best to buy inexpensive, yellow poly line which floats. You would want to avoid a sinking rope in most situations.
It depends a lot on current, depth and wave action as to what will work well and what might be a problem.