2 weeks untill the leap

I would have 3 if I had a lot of samples as they take up a lot of space more than anything. You don't sound like you have that so there's not a lot of point. Adding a third harddrive is inexpensive {well it was before the massive flood inflated prices} so you can do that in future. It's easy to move and re-direct samples.

it also depends on how much memory you have.  Windows automagically moves things in and out of memory as you use them, behind the scenes, so if you have enough mem to stow all your samples (all that you are using in a particular song or project), then the extra HD is less important maybe.  But like that handsome guy said, the disk isn't too much to add unless your budget is really taxed. If you have to choose between them, choose more memory.  How much will you have in there, BTW?