I actually went through this exercise recently with Cogent, Zayo, and two other providers. The requests were all made via email at roughly the same time. HE was by far the quickest (I think under an hour), with Cogent being about half a day initially (but they did miss a BGP session, which was fixed within a few hours of notifying them), and Zayo taking about 3 days, with a follow up call around the 2 day mark.

From an outage standpoint: I've had three outages with Zayo, the first being the most painful (left hand doesn't talk to right hand), the second was brief and they provided an RFO same-day, and the third being similar to the first, but resolved quicker because I was able to reference details from the first. I've never had total outages with Cogent on my transit, but I have on transport, and they were relatively quick to respond, resolve, or provide details from third-party providers each time. From a quality standpoint, I "feel" like the Zayo transit is better, but maybe that's because I pay more for it. I think from a peering standpoint, I tend to see better paths through Zayo. I've seen Cogent send traffic way out of region for several content providers - causing customers to complain about high latency to Google.