I suggest your upper floor.
If you can put the antenna outside, you wont have much with RF issues. As long as you use a balanced antenna, you shouldn't have RFI issues.
Grounding, could be an issue. However, if you take a look here: http://www.w8ji.com/house_ground_layouts.htm
You will see that second floor grounding issues can be resolved. Common entrance for cabling, bonded grounds, no ground loops, and a good electrical ground will all set you up with a great set up.
Also, above ground floors, you wont need a hammer drill to put cabling through the wall
I have a second floor ham shack. There are some deficiencies in my set up (Which I'm getting around to correct), but they are not insurmountable, nor difficult to resolve (Except the electrical ground, which requires a new power line to get pulled though a wall).
So, put it where it's most comfortable to operate. If you end up putting it in a "complicated" spot, that just means you'll quickly become an expert in resolving grounding (RF and Electrical) issues. More often than most, you'll be chasing ground loops and equipment isolation anyways.