Even if you assume multiple 3G carriers are available at the conference site, 3G bandwidth is much more scarce.

So, although 3G does enjoy better dynamic resource allocation (centrally managed using sophisticated algorithms), the overall experience will be expensive and the bandwidth per user will be lower.

A possible solution is carefully placing a few pico-cells in the conference area. Still, a properly tuned commercial-grade Wifi network will almost certainly outperform any 3G setup.