The PTI is a great super heavy card stock for bases. The white seems bright enough but I don't like mixing it with my Stampin'Up Whisper White as the layers. There is a noticeable difference ( at least to my eye ). The whites just don't "match"....the SU layer ends up making the PTI base look like it has a little bit of gray in it.
I have the GinaK Designs white in the layering weight, and it seems closer in color to the SU white....I've been wanting to try the heavy base weight. I have some of GinaK colored card stock and it is very heavy and sturdy for bases and is listed at a lower weight than the white.
I think either of these would make you happy. The only drawback is the price. If you are making large quantities of cards for Operation Write Home it can seem expensive. I make cards for OWH but not as many as I could if the paper cost per sheet was lower.
Mary in NC