Gary, what will your top shutter speeds be like in ULF. 1/25 or longer? If so then I'd go for the fp4+. Consider that under sunny-16 light you'd not be able to shutter fast enough to use the hp5+ at apertures larger than f/64, and that'd be a waste of the resolution of your film and probably also your lens.
As for grain... non issue at 8x10 already IMHO.