Personally, if you are building your own I would go with Win8, simply for the more lenient System Builder Personal Use License license that lets you install it on other machines or heavily upgraded machines without worrying about whether it will treat it as another install.
Win7 technically only allows OEM install on a single machine, and if you upgrade the motherboard, or do enough system upgrades, you might be SOL.
Besides, if the start menu things bugs you you can always get it back with Start8 or StartisBack...
64-bit, whichever you go with. You can always use Start8 to get Win 8 to look more like Win 7 if you don't like the tiles.