If you have a valid license for 7, use it. If you have to buy a license, it depends. 7 will be supported for almost four years to come, 10 has no schedule. Then again, 7 is mature whereas 10 still has some teething problems - and will get some with every major update where the entire OS gets upgraded for new features. That said, 10 has lots of potential and it can use resources more efficiently than 7. It's a question of preferences and liking.
The same can be said about hard disks. If you need lots of storage, get a mechanical since the price per GB is much lower. If you can't wait for the OS and your programs starting, an SSD will save you a few minutes daily. Note that an SSD won't affect your game FPS in any way. Again, SSD's are the future, spinners are matured technology. Since we can't save the saved minutes to be used when the grim reaper knocks on the door, I prefer the mechanical drive and use the longer starting time for getting some beverages or to get rid of them.