Tri-X is more "gritty" to my eye; I also think it has slightly higher contrast in the mid-tones, which is very nice for printing directly on medium grade paper. When pushed, it loses tonal reproduction and grain quality quicker than HP5, for instance. This can be put to use if you want a certain grittyness with high contrast effect. I use it only at box speed, and for me it is a wonderful film with very nice character for portraits, photojournalism type stuff etc. I am not sure I would use it for landscapes or architecture. And I tend not to use it in direct sunshine.
Originally Posted by Arctic amateur
HP5+ is slightly "smoother" to my eye, and takes pushing with more grace. The difference is small, and I wouldn't be able to always tell which is which based on two prints of different subject matter. But when I put two images of the same scene on the different films next to each other, I think I'd be able to tell the difference. Especially with skin, the HP5+ looks slightly "boring", which again is a way of saying almost nothing :). I just prefer Tri-X at box speed. At 1600 or for direct sunlight I'd prefer the HP5+, though.
I have stopped using Kentmere 400 / Rollei RPX 400, since it doesn't like Rodinal, which is my standard developer. I do have 20 rolls or so of RPX in 120, that I will develop in HC-110, but apart from that, I don't see myself stocking up on it again. I have shot quite a bit of Neopan 400 in 35mm, and I really like it. Since it is unavailable in 120, I won't restock. It is a matter of standardisation, as well as passive protest at Fuji for not making this film available in 120.
When you compare "same class" films, you will find differences to be rather academic. TMax 100, Acros and Delta 100 are indeed very close. But they have slight differences in some parts of the tone curve, that may or may not interest you much. AFAIK, the TMax curve has a gentler shoulder, while Acros has a distinct lift which makes the latter slightly more prone to blow out highlights. If you are a wedding photographer who sees a lot of white dresses every day, then you might consider TMax instead of Acros. In my usage Acros is just fine, and since it is the more affordable of the two, I have more or less standardised on it for slow film.
T-grain films are less forgiving of development errors of time or temperature. They have higher "gain", which means that small changes in time or temperature have large effects in negative density. For beginners, I'd recommend FP4+ and HP5+ with more ease than TMax, Delta etc, as they are more likely to get useable negatives. The conventional films also record small tonal variations slightly better, which is why despite the grain, prints from them can indeed look more interesting and more "alive" than the T-grain films. But again, this is really subtle, and it takes good printing hardware and technique to show such differences. On the average web-sized scan, this will be totally irrelevant. If grain is all that worries you, then it is obvious which films would give you better results.