...it will also depend on which developer you use. Since every person who has replied likely uses different agitation methods, developer type, concentration, temperature and container types, the results will vary. My experience with Delta 100 has been favorable for fine-art prints and portraiture, but I don't like it for general use or street photography because with my methods, way of shooting, developing techniques, etc. shadow detail has fallen off the map and the highlights hit too vibrantly and my mid-tones get lost in shadows.
But since we're talking about 8x10 contact prints...you'll notice nothing in difference in terms of grain -believe me. I can't see grain in fp4 or delta with a 35mm neg blown up to 8x10 to the point where I have to try to focus with a grain focuser until my back hurts trying to find any of the stuff. I'll give you one of my kidneys if you can prove otherwise.
...really what it should boil down to is characteristics, not grain, which is what you were kind of originally asking. And I would say that FP4 has a more classic look with a longer range of midtones and better controlled highlights, where Delta 100 has a more "digital" feel to it, where it's seemingly more crisp and sharp. I prefer FP4 for this reason, but that's just my opinion. Unfortunately, you'll likely have to buy a box of each and check it out for yourself.