SSD Boot time = 15.6 seconds
7200 rpm enthusiast HD Boot Time = 21.2 seconds
SSD Boot time = 16.5 seconds
Those were recorded on our test box which has (2) SSDs, (2) SSHDs and a HD. System had fresh install of OS installed on the three different storage devices and which one was booted was determined by BIOS selection of boot device. With games AAA titles going from 40 to 95 GB these days, SSD storage of games is not feasible for most budgets.
My recommendation would be the smaller 240 GB SSD SSD with a 2 TB 73200 rpm SSHD.
That being said, the advantages of each in its role as an NAS will be gimped by network speed which since you described it as an NAS, it's not the speed of the storage device which will determine loading speed, but the ability of the data to travel over the network.
NAS however is not really the correct term if I am understanding your planned usage. For example, the box I am typing from a CAD Workstation in my Home Office. It also serves as:
a) CAD Workstation make great gaming boxes as GTX cards lead the pack in both usages.
b) Office network storage. All the office boxes have their files stored on this PC
c) Home network storage. All the home boxes do their data backups to this PC
d) Media Server. All movies, music etc is stored on this PC for both office and home users.
Basically ... an NAS is a storage only device with no ability to do anything itself with the files stored upon it. In essence, your Gaming PC is also serving as a Data Server.
I would suggest:
SSD to boot your OS and Programs
SSHD to store your Games, Data
Eventually a 2nd SSD to store backups of the other 2 in case one goes south.