Both have the same 2 year warranty, just shop around for the best deal.
Remember to check shipping costs, it can a tenner or more to the total!
A few sites: Overclockers, Scan, CCL, Misco, Dabs, Microdirect, Ebuyer Pixmania and Eclipse Computers are usually good.
Yeah essentially at low resolution, the stronger core comes out, but increase the resolution and the HD5770 drops off a bit, but would still be playable. For a longer term strategy I would recommend not bothering with the Crossfire solution, but re-investing in a new card when the architecture changes next year, which would give you better performance than two HD5770 in Xfire which shows a lot of limitations in heavy workloads.
HD5770 should be nore than enough for now, and give you the savings for later.
As for Mfr, I prefer Sapphire over Powercolor, but I find other like HIS, MSI and Gigabyte to be better build quality usually, but if those are your only 2 options, then Sapphire IMO.