Don't be thrown by the complexities of all the different Wi-Fi pricing models. It's not really as complicated as it might seem.
As a practical matter, the first time you use a Wi-Fi hotspot, you'll probably take advantage of a promotional offer, or buy one-time (or pay as you go) access. It's a good idea to stay uncommitted for a while and to try a variety of different networks.
So start with one-off usage, and get a feeling for a number of the Wi-Fi hotspot networks. You should make note of locations, access speeds, and how good the customer service is.
After you've used a number of Wi-Fi hotspots, you should begin to get the sense of your usage patterns, and you might be ready to sign up for an extended payment plan by the month (or even an annual contract). If you keep a log showing your actual usage and compare it to pricing explained in this section, you might be able to come up with the best pricing comparison.
If a particular location is vital to you, you might be stuck with one particular provider, and comparisons won't matter so much.
The roaming features offered by Boingo and (to some degree) Cingular Wireless make these networks possibly more attractive than the others.
It's hard to beat the massive coverage of T-Mobile Hotspot with all those Starbucks locations. In other words, if you can afford the T-Mobile fees, they are probably the best choice going. However, if you are primarily a business traveler, this might not help you as much as availability in airports and hotels.
From a customer service viewpoint, you might be happier with one of the networks whose primary business is Wi-FiBoingo, T-Mobile, and Wayportas opposed to one of the old-line companies for whom it is an afterthought (meaning Cingular Wireless and Sprint).