The PR3 (B16) and P72 (GSR) heads flow essentially the same.

Now, stock-for-stock the PR3 has a higher volume which means it produces a slightly lower CR than the P72 assuming the two heads are installed on the same block. If you're going to have the head ported, the larger surface area of the PR3 head will give a bit more flexibility when playing around with stuff but either head can be made to breathe well.

If you're buying a second head just to limit downtime, keep in mind that the PR3 and P72 intake manifold flanges are different so at the very least you'd need a stock B16 manifold to go with the head. Additionally if you're using the GSR ECU, it will be a bit confused by no longer having the two-stage intake manifold.

I guess the main thing to think about however, is how long you plan on continuing to drive around without the JRSC.