while i dont mind sli, my next build is in a lian li case with only 2 expansion slots. only room for one card. my computer gets moved too much for it to be a full atx tower (thus why i shelled out extra for a case with a handle on it.) i plan to watercool the gpu, so the exaust will not be an issue.
I would go with the 690 because it has better performance, will sell for more in the future, and will play future games better. However, it's easy to spend other people's money. So if you have the budget and feel comfortable doing so, I would get the 690.