Remember that although some clients may not have all the features you are looking for, you may be able to get them from your other office applications. For example, if you prefer Netscape or Eudora, but still want an advanced scheduler, use Microsoft Scheduler, part of the MS Office suite, or a similar product, and stick with the mail program you like. Consider Technical And Customer Support. By outsourcing your e-mail, you're at the mercy of your ISP's server. If you're having e-mail problems, you want a real person to talk to.
Some clients provide extensive, free support via telephone, Internet and e-mail.
Also check the quality of the clients' online tutorials. While e-mail programs are pretty straightforward, it can be hard to find some of the more obscure but useful features, especially in sophisticated programs. These could be deciding factors if you're on the fence about two products.