Oddly enough I'm looking at a few 4tb hdd's to get started. I was looking at the WD Reds which are going for $208 each. I just found a Seagate SSHD 4tb going for $175. Same 3 year warranty.  Only con I can think of is it's a 7200rpm which would be a bit more for power but probably not enough to worry about.

 

Would unraid utilize the 8gb NAND RAM?

The SSHD won't provide any notable benefit, given the typical use case for UnRAID ... but it won't hurt anything.    7200rpm means it'll have a faster sustained transfer rate than the WD Reds, since they both have 1TB platters, but the Seagate spins about 33% faster.

 

In short, it will perform very nicely, but it's unlikely the SSHD will add anything noticeable to the performance.

 

Most of these devices automatically cache the most used blocks without any need for the OS to know, as Gary says,  typically this wont make a great deal of difference to the average unraid use case as you don't access the same files again and again.