Should I get the 2.5 inch or the 3.5 one?

If you have the space and mountings in the case then 3.5 naturally. larger drive offers more bang for the buck and that's the most important part when thinking about bulk storage after, storage size and reliability. Things like reliability, noise, write and read speed are more of a question about the model and manufacturer, not the drive dimensions. IMO things like energy consumption when thinking about hard-drives in a PC build are irrelevant, but there 2.5 inch might shine.

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:

1. Get an HDD with a large write cache.
2. 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.)
3. 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.

I'd say 3.5, easier to find a CMR disk which you want for a NAS especially if you seed. I suggest you buy two if you can for a backup. But keep in mind HDD optimal temperature is between 12 and 45, it took me 2x140mm fans running at 1500 rpm to keep at 40%

2.5" is almost certainly SMR, which you don't want.

Lol you should get a M.2 then.

All other attributes being equal, it doesn't make a difference.