It depends on what's most important to you.
ACT is more mature and specifically targeted at sales force automation
and has more features than BCM. On the other hand BCM is integrated
directly into Outlook while ACT is a separate application that can
synchronize some data with Outlook.
Users who already use Outlook and are looking for more CRM features
should definately try out BCM.
If you already have ACT, but spend most of your time in Outlook
communiacting with customers, then you may find BCM is a better fit for
you.
If there are specific features in ACT, like custom forms, that you
can't live without, then you should let Microsoft know what those
features are. They regularly study all feature requests and rank them
before deciding what features to add next.
For v2 of BCM they decided to add database sharing for multiple users,
integration with accounting, and PDA.
I expect custom fields and forms are high on their list of things to do.