You are mixing rubber and rubber, right? If that is the case, you should be fine and should not be able to tell a difference. Someone could easily imagine that they notice a difference because they know that the bushings came from 2 different sources.
If you are mixing rubber and poly, you will not die, but you could notice a difference. Poly will be firmer and also articulates more freely. Poly bushings actually rotate between the inside diameter and the inside sleeve. Rubber bushings on the other hand do not rotate about either, the ruber bushing itself twists. The resistance to rotation of the arm due to bushing is small compared to the moment caused by the spring force, so the firmness of poly should be the only noticeable difference.
If you are mixing rubber and poly, at least energy suspension poly bushings will require that you use the energy suspension outer sleeve also, the stock outer sleeves are thinner material making the ES poly bushings loose in the stock outer sleeves.
Tim