Verizon has been much less aggressive with Wi-Fi, though it does offer some hotspots. So Verizon will probably rely more on its cellular network to meet any surge in demand from the iPhone, Marshall believes.

AT&T's use of Wi-Fi presents some trade-offs, Marshall said. Wi-Fi can be easier to deploy and doesn't require licensed spectrum, but it gives the carrier less control, he said. Wi-Fi networks have fewer mechanisms for capacity management by a carrier. As a result, when AT&T shifts a user from cellular to public Wi-Fi, the iPhone user's connection may be more susceptible to slowdowns when more users jump on the Wi-Fi network, Marshall said. On the other hand, Verizon may have more work to do if it wants to keep building up its cellular network to keep up with demand.

Over the long term, a few factors could help Verizon do that job, analysts said. As users buy LTE devices, the burden of their data traffic bypasses the 3G network. Small base stations such as femtocells, in homes and other places with high demand, could pick up the slack. And the FCC has shown an interest in freeing up more spectrum for expanding the capacity of cellular networks.

Entner, of Recon Analytics, said Verizon's network should be fine in the short run.

"In the long run, it all depends on how much money they put behind it," Entner said.