"A carrier Wi-Fi network should always be better than using a cellular Mi-Fi or air card," he said. "But I can't speak for every situation. Perhaps there are bottlenecks. But a well-engineered carrier network should deliver better performance. Air cards and Mi-Fi's are better used in places where Wi-Fi isn't available at all."
Of course, says the guy who's company builds WiFi infrastructure for trains like Acela. No way. My own MiFi is going to give me a dedicated 3G connection that I don't have to share with the kid behind me watching netflix.