GTX Titan vs. Tesla K20/K20x

one would think the K20s would be better.

So from my reading on this subject, my conclusion is that the performance difference between the Teslas and the Titan will be marginal.

Biggest difference will be the lack of ECC ram on the Titan, resulting in occasional flipped bits in the results, as compared to the Titan's, but since folding at home is a project intended to primarily be run on consumer hardware, they have to have designed their algorithms to duplicate results to find false ones due to flipped bits ayway, so the Tesla would likely not benefit at all from this for Folding at Home.

For scientific work in a controlled environment - however - the ECC on the Teslas would likely have a huge advantage.
To be honest, now that the GTX 780 is down to $500, you'd be better off just getting two of them instead of a Titan, or six of them instead of a single K20.
Thanks for all the info.

I'm considering getting Titans or 780s. Will see which I can get the best deal on.
I believe what you want is the Quadro K6000

from Anandtech
With a quoted compute performance of 5.2TFLOPs/sec for single precision (FP32), Quadro K6000 is the first fully enabled GK110 part, shipping with all 15 SMXes up and running. These in turn are clocked at roughly 900MHz, the highest base clockspeed for any GK110 card. Paired with this GPU is a staggering 12GB of VRAM clocked at 6GHz, marking the first NVIDIA product shipping with 4Gb GDDR5. The TDP on all of this? Only 225W.
Just adding some tidbits to the necrothredage:

My air Asus Titan will OC to only about 1090 on graphics loads but will push to 1.242 on folding at home. I don't think it's using all five blocks to do this and haven't asked but it is clearly NOT running some of the components that fAh doesn't use. If I try to start graphics at that speed it crashes.

So for programs like fAh I don't think it will make a difference.

The main difference between the T and the T is the T is designed for a 100% production duty cycle, it has a superior heatsink and noise is not an issue. I'd LOVE to have 2x the airflow over my Titan, it dumps at 82°C when I push it for folding.
if you're doing CUDA stuff and can afford it you might as well go tesla because they haven't been castrated like the mainstream (sic) gamer targeted Titan.

titan wasnt targetted purely at gamers, if it was the case it would have fallen flat at its price. it was targetted at the exact opposite.
You can't really compare them, there is a lot more to the K20 than just the card. Different drivers/software etc.

AS for the various applications, well, try them and find out.

a 780 and a 7970 use different drivers and you can compare them?
Now that the titan black is out it might be the one to get for cuda