Better to do what ? What resolution ?
Both cards are capable of playing most DX9/DX10 games very well at 1920 x 1200/1080 resolution but the 260, as well as the 4870, is about 10% faster than the the 5770. Neither card can handle Dx11 at this resolution, the 260 because it's not designed for DX11 and the 5770 because it doesn't have the bandwidth to handle the load. At lower resolutions, you'll have to weigh the performance drop versus DX11 support. http://www.anandtech.com/video/showdoc.aspx?i=3658&p=14
Like nVidia's 240, the 5770 is a card w/o a market that makes sense at it's current price range. If you wanna do DX11, go w/ the 58xx series at HD resolutions and up.
OTOH, the 260's price has been driven up about $40 in the last month so frankly, I have a hard time recommending either at this point.