Hi John,(posted 8902 days ago)I haven't shot a lot of FP4, but I'll throw my two cents in here:
HP5+ and Delta 100 would probably be my choices if you want two seriously different emultions. I *really* like Delta 100 (at ISO 80)in PMK, most of my meduim format B&W is that exact combination. I shoot mostly nature, but I'd recommend giving Delta 100 a try with PMK, despite the T-grain emultion, I find that PMK gives a wonderful look. FP4 is a traditional emultion like HP5+, if you're going to throw another film in the bag, I'd recommend going with something that gives a different look.
I don't think the staining is significantly different in different speed emultions, PMK is almost all I use these days, and I've shot everything from Pan-F to Delta 3200 with it. I don't think Pan-F shows as much effect as the Delta emultions. HP5+ stains really well in PMK, and it's definitely worth using.
Paul