Personally I've never seen the appeal of SLI, unless you're running more than two monitors.
If it's one new-gen vs two old-gen, for me it's a no-brainer. Sell your old-gen and use the money against the new-gen. Adding another old-gen means that you probably won't get anything for either of them when you next upgrade (because you hang on to old tech for longer). You also don't get the advantages from any improvements (hardware as in die-shrink; software as in new API).
SLI isn't guaranteed to work well (or at all).
If it's 2x current-gen vs 1x current-gen, then only the bit above applies. But you're still going to use more power, generate more noise and heat, spend more time faffing with drivers,
SLI makes sense from a manufacturing point-of-view. It encourages people to hold on to their old card and buy a new one to SLI them. That means more sales of brand new cards, which is great for them. But I don't think it's really all that great from a consumer point of view.
Unless, that is, you want to buy 2x top-of-the-range cards, in which case money isn't really a concern.