I have used both Southwest's (Row 44) and Delta/Airtran/American's (Gogo) service on more than 100 flights on each carrier over the past two years. I am a A-List Preferred flyer on Southwest and a Diamond on Delta; both airlines are great at what they do and I use them for specific purposes.
Southwest's wifi product is horrible and not worth the $5 fee. On a technical basis, their implementation is much too convoluted. Southwest uses Row 44 and they use satellites mounted on the roof of the airplane and they integrate it with traditional HughesNet satellite Internet service. Additionally, they have implemented a WAN link optimization tool to try and get more out of the connection which frankly creates more problems than it solves. On top of all of this, they put an injected banner on every web page to market Southwest, the flight status and the wifi product. My corporate VPN has issues with their service - sometimes it works, sometimes it does not. Speeds are completely unpredictable, service is unreliable (up/down during most flights even in level flight) , applications that operate over port 80 but do not use a web browser such as iTunes, other web apps that don't like frames content, etc. can't deal with the injected banner. Southwest's approach requires substantial changes to the roof of the plane to install a satellite dish and that is probably the reason that their deployment has taken so long.
Delta/Airtran/American's product is awesome and well worth the ~$10 or monthly fee. Gogo uses terrestrial (ground) stations and equipment mounted on the bottom of the plane to deliver their service. Speed is impressive (you can watch Youtube with a pretty similar experience to what I have on my cable modem at home) and reliability is very high. I have had a few spots (west of Minneapolis, near Reno and -in the past- near Atlanta) where service became spotty, but in flying 250k+ elite miles on Delta and lots of random trips on Airtran and American I have maybe had 1-2 mixed experiences. After you authenticate to the Gogo portal (like at a hotel), they don't mess with your session at all except to block apps that they disclose in advance (Netflix, voice services, etc.). Delta/Airtran/American have been able to rapidly roll out this service as it does not make many changes to the airplane or its aerodynamics, and thus I believe that service can be rolled out in two overnight maintenance trips.
Southwest's solution makes you feel like you are making an international long distance call in the 70s/80s, Delta/Airtran/American is like you are on DSL at home.
That all being said, Southwest's choice of satellite is not ground station based and thus it has the potential to be used well offshore and on intercontinental routes. Two issues with this: 1) they don't fly either place and 2) their choice of HughesNet doesn't have coverage despite the technology itself theoretically working. Delta (Gogo) are working on introducing satellite-based service on intercontinental routes in 2013 and we'll have to see if they end up like what I experience with Southwest on a domestic flight given the change in technology. I certainly hope not!