I would say that FP4+ has inherently more contrast than HP5+. I would probably try Delta 100 for a medium speed emulsion to use alongside HP5+. Trying to be objective here. To me FP4+ is more like Tri-X in how it renders contrast, that is - more of it!
That can, of course, be cured by changing your development regimen to a degree. I usually dilute my developer more when I process FP4+ to cut contrast.