depends on what Mac you are plugging it into. In a 2006 or 2007 model, you can't tell the difference.
Both draw 2-D shapes at about the same speed, it is only when you are doing fancy stuff that the 5870 pulls ahead.
The 5870 takes two of the two aux power connectors, and costs US$450 rather than the 5770 that takes one and costs US$250.
you didn't say you have the 2012 Mac Pro, and at least that allows you to use a range of graphic cards with 10.8.3 and above. A 2008 can as well but does not benefit as much either if you looked at www.barefeats.com test results.
so it is why and why you need to or looking at one, maybe more VRAM more than anything? and what your budget is. Even if you think $450 is reasonable, there are other things that will buy to improve overall system performance, like SSD for system and scratch, more RAM, more disk drives.
Is better but for what ? I asked.
You are low on memory, 6GB is minimal for these systems.
Barefeats did a lot of testing of various cards and apps to help people.
For $449 it may not be the best investment or pay off in running apps or system better at all. The 5770 does quite well for most.
If gaming then of course a $599 EVGA Mac Edition GTX 680 would probably be 'best.'
September 13th, 2013 -- CRAZY FAST GPUs for Mac Pro: GeForce GTX Titan and 780
May 30th, 2013 -- Tandem GPUs = Faster Pro App Rendering. (Features Octane and After Effects rendering with dual GPUs; added DaVinci Resolve on June 5th.)
May 10th, 2013 -- Do the newest GPUs run faster on the 2010 Mac Pro than on the 2009 and 2008 Mac Pro?
April 26th, 2013 -- PART THREE: EVGA GeForce GTX 680 Mac Edition versus the 'Sharks' (insane GPUs like the GeForce GTX 580 Classified and GTX 690)
April 18th, 2013 -- PART TWO: EVGA GeForce GTX 680 "Mac Edition" Versus Past and Present Mac GPUs running Pro Apps
April 16th, 2013 -- PART ONE: EVGA GeForce GTX 680 "Mac Edition" versus Past and Present Mac GPUs running OpenGL accelerated Games and Benchmarks
April 5th, 2013 (Appended) -- SHOOTOUT WITH THE 'SHARKS': Sapphire Radeon HD 7950 for Mac versus "big fish" GPUs (Added full results for the Quadro K5000 and a flashed Radeon HD 7970.)
March 22nd, 2013 -- PART DEUX: Sapphire Radeon HD 7950 versus other Mac Pro GPUs (running StarCraft, Diablo, Team Fortress, FCPX, Premiere Pro, Photoshop, and LuxMark)
March 19th, 2013 -- REVIEW: Sapphire Radeon HD 7950 versus other Mac Pro GPUs (running DaVinci Resolve, Motion, OceanWave, Heaven 4, Valley, Civilization, and Dirt)
March 12th, 2013 -- The Current State of Mac Pro GPUs -- before the next "shoe" drops -- new benchmarks compare the current 'crop'