I have one those ASUS ROG laptops, so it already uses a 128gb SSD to boot up. It already has the HGST-brand 1TB HDD for storage; and I was thinking of getting the Seagate SSHD also to store stuff. Would that SSHD better, in any sense?
If you're talking about the WD 2.5" hybrid drive that include a 120GB SSD, that sort of thing has uses, but won't benefit you that much.
If it's some other "SSHD," you're spending money for a questionable benefit, also.
How much RAM does this ROG laptop have? For $30, you might want to use something like this:
http://www.romexsoftware.com/en-us/primo-cache/
If 8GB RAM, you could use 1GB to cache the HDD. If 16GB, the same or more, and you wouldn't miss the RAM it uses.
Fact is, and I'll take flak for this -- you could clone your SSD to the 1TB HDD after backing up the latter, transfer the file-folders you backed up to the now-bootable HDD, delete the SSD partition, cache the entire HDD to the SSD, and cache the whole enchilada to RAM -- all with the same software.
Aside from the flak I expect, I've had PC running on three PC's for about a year now, and it hasn't missed a lick. One of those systems is a laptop -- sleep and hibernate-enabled. Works great for me.