Personally I would rather use a single card over the crossfire or SLI performance matching pair of GPU's
The dual card option really only comes into it for me when you are talking about top end performance.
For instance if I wanted HD 7970 or GTX 680 type performance I would rather get a pair of GXT 560Ti cards in SLI because of the cost saving.
However if I wanted HD 6970 performance I would not get 2 GTX 550 cards I would get the single card.
There are less and less actual Issues associated with dual cards these days but you do sometimes have to wait for support for new games which means you sometimes have to drop to a single GPU to play them.