Windows Mobile provides a number of benefits since it’s deeply integrated with Exchange. This includes management for administrators, throttling policies, and enhanced device policies (storage encryption for example). Also Exchange ActiveSync is included in the Exchange Standard CAL so more than likely all of your users are already licensed for EAS so you don’t need to spend extra on additional licenses.
“Better” is a subjective term here. It depends on the needs of your users. Yes going windows mobile keeps everything in the same family and integrates well of course, but some companies prefer the blackberry for security reasons. So it all depends on what is most important to your corporation.