Couple of suggestions based upon my journey down the road of 12x20.

First thing I would do is to actually acquire some small quantities of FP4+ and HP5 in either rolls or say 4x5 sheets and do some small scale printing for various trials of expanding development and normal development to see how the film responds on a smaller less costly scale. Evaluate the negatives over a light table closely and do your best to make some prints with these. Even 4x5 contacts are remarkably vibrant when executed properly. Take good notes and keep the costs at a minimum.

That said if all you were doing would be silver printing either film would be likely adequate and HP5 will shine as the extra film speed comes in handy.

Any of the alt processes are challenging for HP5 because the film curve at the top end turns flat and your ability to generate a net density to hand off to your prints becomes inhibited. This is not as much the case with FP4+. If you conduct your visual print beta test above this tendency will become very visually apparent.

Last thing I would suggest that you do with your new 12x20 is to make yourself a scale for your lenses that provide you with a reference point for infinity and a scale beyond infinity at measurable points so that you can make sure that you check for any bellows extension corrections that need to be dialed in. These big cameras regularly get into bellows corrections with photographs that one would not intuitively even check for these issues - but they are there. Ask me or anyone that shoots these formats how they know? The scale provides a quick reminder to add exposure to keep you on the straight and narrow. Sheets this size are much to expensive to unintentionally leave under exposed and then have to resort to over development as the last man standing.

If you have small light leaks at the flap edge of your holders, use some thin black fiber tape to cover the flap seam on both sides of the flap on the top surface of the holder.

Good Luck!