Depends on your existing platform for communications. If you run Exchange for instance, anything that is server acthivesync compatible is great as you won't need any 3rd party server systems injected in your existing platform.
Most phones supporting it that is not running Windows Mobile might have some issues though, and even if you would standardize on WinMo there's a lot of models and as with desktops and laptops, a support organisation should only need to support a very limited amount of brands and models. You need to get to testing stuff out to see which feels best for your needs. Personally I've shunned Sony Ericsson and Nokia, adapted WinMo because the managability with existing Windows server systems, but I haven't tried Blackberry at all (not a big hit in Europe so far).