Seesmic's offerings of Twitter clients. I started out by using the Air application, Seesmic Desktop. I moved on to their .NET based application, Seesmic Desktop for Windows. The reason I changed was mostly because of the resources that the AIR application took up. The .NET application is much faster and takes up a lot less resources. I found Seesmic desktop for Windows a really nice app and used it for some time (it's still on my computer). But the one thing that made me switch is the portability of Hootsuite.
Hootsuite is web based, so whether I'm in my regular location, at home or on the go, my settings are EXACTLY the same. With Seesmic I had to transport a settings file with me and install the software on each machine I was going to use it on. With Hootsuite, since it's web based there is no need for that. Yes I know, other apps including Seesmic have a web based client. But Hootsuite has done a really nice job on theirs. You can have multiple accounts, as well as multiple people tweeting with different access depending on what you want them to do. It's like combining the best web twitter clients with something like cotweet.com.
In Hootsuite I have multiple accounts setup for twitter as well as LinkedIn, Facebook, and Foursquare. I have many columns in each twitter account which includes lists I follow and searches. Searches are a problem I found with Seesmic, they weren't updating correctly. I also found that Seesmic lists didn't update correctly in their .NET client. I reported both these problems to Seesmic, and although their team is very good, it seems they were concentrating more on mobile applications than the desktop clients.