A lot depends on your requirements (and age).
The BlackBerry is much more suited to people who need secure, wireless e-mail, with web browsing, MMS, SMS and phone. And the major strength is e-mail.
The Sidekicks appeal to a younger demographic, which may be more appealing.
And I have never seen an IM device that guarantees "Instant Message And Text WITHOUT FLAW". Since IM seems important, perhaps the Sidekick is right for you.