It really depends on your usage model.  The 5770 is currently the best (readily) available performer when comparing power consumption and initial costs.  If you're looking for the most performance you can cram in the least available space and the up front money isn't a huge concern, go for something like the 6970.

Using the bitcoin profitability calculator and just changing the hashrate, cost, and power use of the video cards (not including the rest of the PC costs/consumption), the 5770 @ stock shows a $1.67USD/day profit with a 66 day "break even" (the card paying for itself).  The 6970 shows a $3.44/Day profit with a 99 day "break even."  Of course these calculations don't account for the difficulty going up, but it shows that long term, the 6970 is the more profitable card.

Of course, a lot of us little guys can't be dumping $340 here and there, which is where a card like the 5770 shines.  We can only hope that video card manufacturers don't notice these values and adjust their profit margins accordingly.