Pyree :
Get HDD.
Your grandad is probably not a power user or a gamer and he will appreciates the space he can use to store photo, music and video than the fast boot time, and fast application and game load time.
Also consider the hybrid drives.
A HDD + SSD cache setup would probably be his best scenarior in terms of quality computer use.
Gives him the proper amount of space to store, and the proper speeds to feel like he isn't racing to either die or watch OS load (which is how I feel every time I had a OS on my HDD. That has changed since I spent $100 on a Crucial Adrenaline to boost my HDD.)
A SSD would be nice if he knew exactly what to do with it, so a SSD cache makes more sense imo since it delivers wanted speed to the HDD and all of the files on it versus having to decide what programs to put on what which can be daunting if you aren't keen on the software you are using.
I like to look at it as so, I would rather spend my days trying to find more stuff to do rather than spend my days waiting to do stuff. I know some older people like to use the excuse "I don't mind the wait, gives me a chance to do something around the house." Which is fine but for people like my grandma that is a power user even tho she only does a small amount on the pc it takes her longer to let the pc load than it does to get what she needs done.
Normally I would suggest a 120GB SSD and a 500-1TB HDD but as of lately I have been trying to introduce people to SSD caching which is a very great thing to have for mechanical storage drives.
The momentus xt is comparable but I see that the HDD + SSD cache is faster and the SSD cache benifits the HDD more than what the momentus xt's hybrid ssd does. (mainly due to the size of the SSD in both configurations with the momentus XT only having 4gb/8gb of ssd cache versus 50GB of SSD cache plus very easy to setup software that does all the work)
Also if you look at my previous post you will see that the HDD+Adrenaline cache is performing equal to an Intel 320 series SSD which fully saturates its SATA II connection. Compare that to a momentus xt 500gb and you start to see the performance gap between the HDD+SSD cache versus HDD hybrid.
Food for thought