So far this is all I have found about the comparisons of urethane to silicone.

The urethane rubber does have a shorter shelf life. It also deteriorates the more you use the mold, faster than silicone rubber. Urethanes tend to be a more advanced-user material too, so if this is your first venture into mold making, it may prove difficult to work with. In addition to this, even though they are "flexible", bending the mold off the product over and over (even just a few times) allows for the molecules to stretch and break, causing tiny tears pretty early in relation to silicone rubbers (silicones are specifically designed for a longer life over repeated flexing). I started on Urethane rubbers, and once I went to silicone, I was hitting myself for not trying it first.
This is, of course, talking about molds, but it is the same material, so the case can be made for tails too. Basically, it appears to deteriorate faster than silicone (with constant use). Not that I'm discouraging the use of urethanes. I just thought some of you might be curious, as we don't seem to have much in the way of information on urethane rubber.