Robert Jordan dragged out a series for commercial reasons and altered a major storyline when fans guessed it, leading to an uneven story, and then died before he could finish (due to his own slowing down of the series), leading to what most would consider was a lackluster (at best) ending.
Georgerr Martin can't seem to write anymore and let HBO finish his story for him in a slipshod and rushed fashion, poorly handling pacing and resolution for multiple story lines. He's also not in good health, doesn't seem interested in finishing the books at all and will likely die before finishing.
Which author let down his readers more?
As others have said, RJ kept on writing while he was battling a fatal disease. He recited the story's ending twice while almost on his death bed. From what I can tell, at least until he got sick, he had a daily writing routine, and he didn't do too much of the Comic-Con tour, though that was before Cons became so big.
Also, IMO, after listening to the series again this year, I feel even the plot slowdown in CoT was very much in keeping with his style. He set out to write the story of a world coming to grips with the messiah and he meant to do it with as much pathos as he could. The reactions to the cleansing of saidin felt right, when I didn't have to wait 3 years to the next installment.
OTOH, if I try to view Martin empathetically, I think is a textbook perfectionist. He can't seem to mold the series into the shape he wants so he procrastinates on conventions and side projects.
Really a prisoner of his success, because if he was less successful I'd think he should just abandon the series and start new, but how can you abandon one of the most successful series ever ?