Metrocast had a pretty bad reputation five or six years ago, but they've improved over time. They're now one of the few good, small service providers, pretty-much on-par with Comcast and other big guys. I have read some "glowing" reviews, including such accolades as "generally acceptable" (which is a rave from most folks reviewing service providers). There are also reviews saying that their service is occasionally inconsistent.
I've read a few recent reviews from customers saying if that they could switch to Comcast they would (evidently in some areas Metrocast is available but Comcast is not), though. I suspect that folks understand that Metrocast is a smaller provider, and let them get away with things like occasional pixelization and poor HSI throughput, stuff that they'd be much more critical about with a larger company. Folks in the know realize that they could probably get the same level of service from Comcast more reliably perhaps paying a little more, and so it comes down to a value judgment about whether that marginal amount of additional reliability is worth it.
If this hasn't help you decide between your two options, I think you've understood the situation correctly. FWIR, they're pretty even choices.