I think there isn't much of a difference in your case, but, also, 3.5" are generally a better choice than 2.5" in all aspects except portability.
Higher performing HDDs tend to be 3.5", which might have higher RPM (higher read and write speeds), and bigger solid-state cache to speed up writes. I think 3.5's tend to be more fault-tolerant too.
In your case, you probably wont need more than the average read speed. A large write cache could be useful, especially if you have a fast internet: if your cache is too small, your download speed will be capped by the write speed of your HDD. Torrenting writes in random access rather than sequential, and random access writes are quite slow on HDDs.
You can circumvent the write cache problem in several ways:
- Get an HDD with a large write cache.
- Download to a faster drive first, eg. an SSD, and then move it to the HDD. (While it is true that moving from the SSD to the HDD, you would still be capped by the HDD speed, sequential write speeds are usually quite a lot faster than the random access writes that torrenting needs.)
- Use a write cache drive as part of a filesystem. For this, I am most familiar with ZFS, which can have an HDD device combined with an SSD write back device to speed up writes. I think bcachefs can do this too. I dont know if btrfs can do this.
If you are torrenting to the HDD directly, then use a 4K extent size for btrfs or ZFS.