Just wanted to add my 2 cents worth with a huge vote for an SSD. The difference is night and day. I don't think it's just the "15 seconds boot time", but everyone will have experienced what happens after you've had the computer for a while, accumulated a few little applications and programs, and then after you login, you can see the desktop but the computer is basically useless (unresponsive) for a minute or even more because that poor old mechanical HDD is getting slammed and just not coping.
SSD, you turn the machine on, login, launch your game and away you go. The difference is night and day.
If you were on a super-tight budget and desperate to get the best possible FPS, maybe you could drop an SSD. But that's a nice high end build you have there, it would (IMHO) be criminal to cripple it with a mechanical HDD.
Yes, once the game loads the SSD makes no difference (or even once the PC loads if you game is installed on a secondary HDD), but the responsiveness of the system in general is literally night and day.
It is, of course, a personal thing and others will see it differently. But that's my view for what it's worth. Good luck with the build.