Hi udoknowjac,
Wool is a better temperature regulator around a foam material than just a stretch cotton but it can also have more of an effect on the feel of the foam as well.
It would depend entirely on the circumstances. A mattress pad or topper will affect the feel and performance of the mattress so if all you are looking for was temperature control I would probably start by looking at my sheets and bedding and the mattress protector I was using (see post #89 here) so that it had the least possible effect on the mattress in terms of PPP. If you also wanted to change the feel or the pressure relief of the mattress then I would consider a mattress pad (which is thicker than a protector) or a topper which is thicker yet (see post #10 here).
Phoenix