Ever since AT&T and Apple introduced the first iPhone four years ago, the carrier has racked in the cash and customers while selling iPhones by the millions. But along with all that success, AT&T also has received heaps of criticism for a creaking network that has saddled users with dropped calls and slow data speeds.

Though AT&T doesn't deserve all of the blame, it shouldn't escape it completely. Indeed, when I was at CES last week, I couldn't get an iPhone signal anywhere near the Las Vegas Convention Center. It can be pretty miserable, frankly, which is why Phone users and fence-sitters alike have been clamoring for Apple's device to land at Verizon Wireless for years. And now that a Verizon iPhone looks like a sure thing, AT&T will a few defections.

But as even as some consumers switch to Verizon in search of a perceived better network--which is hardly a sure thing, mind you--AT&T still has a few advantages that Verizon can't touch. And these points are nothing to take lately.

A global technology AT&T uses GSM, which is the dominant global cellular technology. Whether you're in Europe, Asia, Africa, Australia, or the Americas, you can find a GSM network for using your iPhone. Verizon, however, uses CDMA, which is present in only a handful of countries outside North America. That list includes Brazil, China, India, and Israel, but you won't find a drop of CDMA across the whole of Europe. Of course, Verizon could close this gap with a dualmode CDMA/GSM device.

Voice and data Currently, CDMA phones can't transmit voice and data at the same time. To you, that means that you won't be able to be on a call and browse the Web or access your e-mail simultaneously. It's a big change for how many people use the iPhone now, especially when you consider that Apple has made such functionality--you can talk on the phone and get directions to Starbucks!--a centerpiece of some of its ad campaigns. The good news is that the CDMA Development Group will fix this limitation sometime during the first half of next year, but AT&T will have a big ball in its court until then.

A faster 3G Despite its troubles, AT&T's HSPA 3G network is theoretically faster than Verizon's EV-DO network. For most people, that has been just a theory, but other users may notice a difference if they live in an area with better AT&T reception. Of course, 3G speed will be irrelevant if Verizon has an LTE handset, but that's an unlikely prospect at this point.

Price If you're an AT&T user still on contract, switching to Verizon could cost you $624. You'll have to pay the carrier's early termination fee (ETF)--as high as $325--and you'll have to buy a completely new iPhone that supports Verizon's network. Though we don't know Verizon's pricing strategy just yet, I imagine Apple will insist on matching AT&T ($199 for the 16GB model and 32GB for the $299 version). Granted, AT&T does prorate the ETF as you serve out your contract, and Verizon could offer an incentive program for AT&T customers, but it's still going to be an expensive proposition.

CNET will bring you live coverage of Verizon's event tomorrow, January 11, at 11 a.m. ET, so be sure to check back for the full story. In the meantime, tell us what you're expecting from the Verizon iPhone.