Any opinions as to which is the better device and why?

My users will be primarily using them for email, they are currently used to
syching their Palm Pilots at the office, so doing that with a Treo will not
be an issue. I know the Treo does not have the same email capabilities as
the BlackBerry, but the extra step having to "Get" emails isn't as big a
deal as the added cost of the data service for the BB. We can save about
$10K annually with the Treo.