Gaming HDD 128 cache or 256?

 The fears of SSD dying out before they became obsolete are unfounded. In normal use a typical HDD (like a Samsung 850 Evo) will last about 100 years before you run up against it's write endurance rating.  So that  " I just dont want to buy a new one every few years." argument is totally false. That 6TB mechanical drive is much more likely to fail before a quality SSD will. 

I've converted mostly to SSD's.  I have a 1TB NVME OS drive.  a 4TB SATA SSD for my Steam library, and another 2TB SATA SSD for non-steam games. And 4TB HDD (w/32GB optane cache) for things where speed doesn't matter (documents, pictures, media etc.) I don't see myself ever buying a mechanical HDD again, save for an external backup.

As for your question; It really doesn't matter. There's not going to be a perceivable difference between a mechanical HDD with a 128MB cache and a 256MB cache.  I would simply see which HDD benches out faster and weigh that with the cost difference (neither is likely to be significant). and go with whatever one you want.  

Hard Drive HEHEHE

You're going to buy a new drive every few years in any case, don't fool yourself.

Apart from that, then simply get a branded SSD (or, if you can, m.2) with a reasonable endurance. As for failures - SSDs are just about as reliable as HDDs. They might fail differently, but fail they will. Which means, if you don't run a backup in either case, you _will_ have a problem at some point.

Writing to HDD also increases its failure rate, as rewriting is normal in HDD use.

So, your starting fact is wrong. Modern SSDs are at least as reliable as HDDs. There is also a combination SSD/HDD you can buy called a hybrid drive.

6TB HDD is generally multiple HDDs in one case.

SSD in Windows and Internet cache make a vast difference.

SSD in a favorite game allows faster load and game settings changes.

Sometimes, you can find ideal settings and then move the game to HDD.

Every modern PC should have an SSD. Their prices have dropped dramatically.

In a 6TB drive, I would be much more concerned about reliability, and 6TB drives are not typically used in gaming.

Cache size does effect speed, but you can just look at I/O data specifications.

All critical data needs to be backed up.