There are a few security improvements, as there pretty much always are, that Windows 7 doesn't have. But given that people are happily sitting with Windows 7, or are overwriting Windows 8 with Windows 7, it's not going to be a monumental difference.
2) There's the option of Dual Booting but would that mean installing all my Program Files twice for each OS? Steam takes up hundreds of GB in downloaded games.
That depends on the program. Since many Windows apps rely on the registry to retain configuration information, you'd end up doing more or less a full install on both sides (just pointing to the same files in a central location). And from that, you can imagine some of the troubles associated...
But if your apps have portable versions (such as versions you could extract to a portable drive), or by default store their information in standalone configuration files, there are probably going to be fewer issues.
Something like Steam would probably be finicky about receiving updates and such, where one side knows it's been updated, but the other doesn't.
I'd personally just go to Windows 7 without bothering to dual-boot.
[also, I nudged the thread up to the Windows 8 forum, from the power8 subforum]