The poly bushings will deflect less so it will make the car more responsive, they are also more durable. The only real draw back to poly bushings is the possibility of binding at the limits of suspension travel that you don't get with rubber bushings since they will twist. Granted you are popping out perfectly good rubber bushings to put in poly ones, but if you are going to do the poly bushings, just do them now. You won't want to pull it back apart again after you get the upper control arms off and back on again.
Former owner of 94 Bird with DOHC 4.6L swapped from a mark VIII
2011 CTS-V Wagon
1968 Torino was an original 289, C4, fully documented numbers matching car. Now: DOHC 4.6L/T-56 swapped, Cobra intake, IMRC delete, BBK long tubes and H pipe. Dynotech Aluminum Driveshaft, 9" rear with TrueTrac and 4.11's.