they trade blows. you can find a 3GB 7970 cheaper than even a 2GB 770. the 4GB can be very close to the price of a 780.
if you can wait for the HD9000 coming out later this year, i suggest you do. if not, i'd go with the 780 for a 144Hz. if neither of those two are within your budget . . . 7970 all the way.