I shoot HP5+ in 35mm, 120, and 4x5.
The differences are easiest for me to see in 35mm, since it tends to get enlarged the most. Discussions of grain and sharpness are less relevant for medium and large format, at least for the amount of enlargement I do (I rarely print anything at all larger than 16x20, 11x14 is more common, and both of those are tiny sections of my pie chart compared to 4x6, 5x7, and 8x10 printing). Everything is plenty sharp and high resolution with low grain by the time I get into 6x6 negs. If you print huge that may not be true for you.
HP5+ at 250, processed with constant agitation in dilute HC-110 (1:50 or 1:63) seems to produce a rather smooth, soft grain, and fairly low acutance. I think it can be a nice look for some subjects.
Here's one and
here's another (these are scanned 8x10 prints on Ilford MG FB glossy from 35mm negs). The softness is not missed focus or a poor lens, these are both shot on 50mm primes stopped down to an intermediate aperture in the neighborhood of f/4 to f/5.6 or thereabouts.
Tonality in large format isn't
anything particularly special, but I don't find
much to complain about either. I can get a good black and a good white. I can retain detail in my highlights with ease. And even though I tend to prefer shooting under lower contrast lighting conditions, which often requires extended development and/or higher grade printing, I rarely struggle to get my tonal range where I want it in a print. Of course, in this format the lower acutance and soft grain are essentially undetectable to me. These are also scanned 8x10s, but from 4x5 negs developed the same as above.
At box speed in XTOL (technically I'm using Instant Mytol), I get fine and unobtrusive grain, with good acutance. I unfortunately haven't scanned any prints made from negatives exposed and developed this way. But it's probably my favorite way to do HP5+ in 35mm, especially when I shoot through a yellow #8 filter, which improves the tonality of most nature photos, to my eye. Everything else I've tried gives me more grain than I personally like. The ability to shoot a 400 speed film at box speed in 35mm and still get a very good 8x10 print that appears sharp and not low-resolution is a huge win in my book.
In general, I prefer FP4+ in all formats when I can use it. I think it gives better shadow separation, I love the way it looks in Pyrocat HD(C) or Rodinal (at least in larger-than-35mm formats), and it strikes a really ideal balance for me of grain/resolution/acutance/tonality. I like Delta 100 in 35mm when I want even more resolution than FP4+ but can't bring a larger format (often the case when backpacking). But you didn't ask about those, and when the extra speed is worth the tradeoff, as it often is, HP5+ is my go-to.