I would go ahead and use the poly blend.
The biggest problem with poly fabrics is that they tend to fray more easily along the cut edges. This isn't a huge problem in a pieced quilt; it's more likely to cause problems with applique.
Another potential problem is that poly cotton is highly unlikely to fade. Over time, cotton fabrics will fade together while the poly remains its original color. Keeping the quilt out of sunlight will slow down the fading.
As for shrinkage, there is so little shrinkage in reasonable quality quilting cottons today (less than 1%, I think), it is unlikely to be a problem. It wouldn't factor into my consideration about using the poly.