Both GPUs are great, but the Radeon HD 7850 definitely edges out the HD 7770 by a decent margin in most games. The better 256-bit memory interface helps in virtually all games - as for clock speeds, that's what overclocking is for!
The HD 7850 gives you a superior memory interface, but lacks the clock speed. If it did have the higher clock speed by default - not only would it absolutely kill the HD 7850, it would use more power. The biggest reason is that it would cannibalize the HD 7770's sales. I would definitely go with the HD 7850!
You can grab Sapphire's Radeon HD 7850 from Newegg for $185 - this is the 2GB GPU variant, and is just $20 more than the 1GB version.