I ran a small, quick study last night comparing the quality of three groups based on a fairly complex task. The task worked like this: a fairly exhaustive list of categories was presented, followed by instructions which said you had to come up with three new categories to add to the list. They could not be duplicates, and they shouldn't be too broad nor too narrow. The pay was $4-5 per hour, but bonuses would be given for good, thoughtful answers, doubling that pay. Here are the results:

Group one: 98% / 10,000 Approved qualifications
They completed all 10 hits in 1 hour and 27 minutes
One worker gave three terrible answers (10%)
Out of all 30 answers, only one was a terrible suggestion (3%)
Out of all 30 answers, six were duplicates of those in the instructions (20%)

Group two: Masters qualification
They completed 6 out of 10 HITs in 13 hours and 30 minutes
Two workers gave three terrible answers (33%)
Out of all 18 answers, three were a terrible suggestion (17%)
Out of all 18 answers, five were duplicates of those in the instructions (28%)

Group three: long-standing private qualification
They completed 5 out of 10 HITs in 13 hours and 30 minutes
NO workers gave three terrible answers (0%)
Out of all 15 answers, NONE were a terrible suggestion (0%)
Out of all 15 answers, three were duplicates of those in the instructions (20%)

So, as you can see, both the private qualification and 98%/10k qualifications outperformed Masters. If you have an easy task and want it done quickly, and are willing to throw away low quality answers, then 98%/10k is a good qualification requirement for your work. If you'd like high quality, thoughtful answers, or have a complex task, a private qualification is likely your best bet.

I didn't include this in my data as it is subjective, but the overall quality of the answers provided by the private qualification were far superior to those from 98%/10k. While the open qualification HITs' answers were original, they were often too broad or too narrow, but you could tell they tried so I didn't rate those answers as "terrible". If you are looking for high quality, nothing beats a private qualification. You can also use someone else's qualification on your HITs, if you use the API, so you don't even have to come up with your own private group. I'd recommend the Elite Proletariat qualification if you want some great workers.