I have used both. Although the Dash does have Wi-fi capability and has Windows Mobile 5.0, unless you desperately need the Windows viewing only capability (Microsoft word and Excel documents), I would go with the Pearl. The Pearl gets e-mail just as fast if not faster than the Dash. I find the menus on the Dash have a slight delay, annoying when you're used to instantaneous access. The Dash has the full qwerty keybord and is spaced nicely. I like the Pearl for reception- slightly better the Dash.
Downside- the Pearls buttons are kinda small and close together. I found myself having to use the point of my finger to type. If your not used to the Sure-type keypad it will take some time to get used to it.