Its really not a question of OR, but AND. You can have the best VGA cooler in the world, if your case becomes a sauna, your card will still run hot. This is even more true for most aftermarket coolers that do not exhaust their heat through a PCI slot, but dump it inside the case. This requires good case ventilation.
That said, I do have an accelero twin turbo on my 5850 and its awesome, BUT.. well, there are serious buts.
Its awesome in the sense that it dropped my GPU temperatures from high 80C to 50C. 55C max when overclocked in a hot ambient room. Its that good. BUT, the accelero only cools the GPU, not the memory, and most importantly, not the VRMs. They include sticky alu fins for that, but they dont stick and are worthless. Throw them away. Instead I used the base plate from the stock cooler. That works fairly well, but if you have a reference card, you will have to modify things a bit. I had to dremel the baseplate a bit to make room for the heatpipes, otherwise the heatpipes would push on the baseplate. You also need to bend a few fins of the accelero to make room for the DVI block. Neither is hard to do, and with a 5870 you dont need to do either I think.
Last point; VRM cooling. Im still not happy with my VRM temps. @850 Mhz the VRMs will heat to 80+C. This is well within spec, but still higher than I like, so I ordered a thermaltake VRM R5 cooler specifically for the VRM. Dont have it yet, but reviews look promising (although I will have to cut my stock cooler baseplate in 2).
Many cards dont support VRM temp monitoring, and I suspect this is the n1 cause for premature failures. With the stock cooler my VRMs went near 100C at stock speed. If you cant monitor your VRM temps, dont assume they are ok.