Are you going by percentage of companies to exit? What data are you using?
Here's a data point: YC took some investment money, which could be indicative of a lot of things. How is TechStars funded? Has that changed lately?
Not much of a data point, for sure, but to really know anything, you'd have to talk to the people involved and get them to reveal information they may well not want to reveal.
TechStars, actually, has a better track record of exits when compared to YC. YC has some bigger companies that are still growing, but the TechStars scorecard right now is looking pretty good.