At least on Comcast, the SB33 is a better modem in my experience. The firmware on the motorola's tend to have more issues in certain areas with certain signal levels. Comcast controls the firmware on the modem, when you connect to their network, they update the firmware and the speed profiles on the modems.
This is very similar to android cell phones in terms of firmware updates. If you buy one through a cell provider, the firmware is updated by the manufacturer and given to the cell provider, but it's up to that cell provider to modify and push the update to your device.
Comcast is extremely Lazy with firmware certifications and updates. They don't spend enough time certifying firmware versions for third party modems once they've made the approved list, unless there's a major issue or something. Firmware updates could take years.
I'm currently using the SB33 after experiencing problems with my MB8611 motorola modem. The SB33 has been rock solid with many months of uptime, I bought a refurb one on amazon for $99.