The 690 will also take two slots but I think that you won't have an issue unless the pci-e 3.0 slots are close together. Usually there is a space in between the full length slots to accomodate the two slot cards. You may only end up covering a x1 slot.
Well... the 690 isn't available yet, so the honest answer is we can't compare the two cards right now. Hopefully it's better than the 590 was in its day. I was unimpressed by both the 6990 and the 590.
The 680 is a beast of a card, though... if you can get them in stock, just grab one. You can always SLI them later