After playing with both Spaz and twhirl I finally have a final vote for which one I will be using on my computers. After all was said and done I hope you find the same results as me. Both seem to have issues in the Linux environment, and here is what I found testing them on Windows and Linux using Ubuntu 8.04 distrubtion. Both are Adobe AIR software which is Adobe's cross platform software development tool. The software will install the same on any operating system that is supported by Adobe AIR.
Spaz can be downloaded and installed from http://funkatron.com/spaz
Spaz I noticed that when you start it for the first time you have to enter your username and password for twitter, but the user name and password boxes appear over all of the settings options. I wasn't sure I was entering the username and password in the correct location. It worked and has some very cool sounds to it, kind of making me think of the old NBC tones that used to play inbetween the tv shows. I got totally spazzed that this program worked better than any of the other clients that were native to linux. The only draw back is every time I restart my computer I had to re-enter my username and password.
Then there is twhirl. My entire body was went into twitter overload. It allows you to cross post to other applications, search through the twitter database to see what others have tweeted about in the past. Say you need to know about a program. You want to see if people like the program. You can search for that program name. Sure enough tweets appear with that program name or what ever you search in for.
Twhirl also allows you to add other twitter accounts god for bid you have more than one twitter account and want to post to more and more accounts at one time. Also you can view and post to your friendfeed account. I guess you can also add multiple friendfeed accounts just like twitter and keep track of many of them.
twhirl can be download at http://www.twhirl.org
After playing with both of them for a week I have totally fallen in love with twhirl it has more features that I like.