Pearl is lighter, smaller device with a better battery. The mogul is great if you like touch screens or Windows Mobile. If you do not want to pay for a data plan and just use an organizer, get the Mogul. While the blackberry plan is $30 for Sprint, you get the most features out of it. Unlimited TeleNav, Unlimited Messaging, Unlimited Email, Unlimited Web Browsing, Sprint TV Basic Unlimited, etc. The Push on the Blackberry is the best in the business, keep that in mind, but it requires the use of a Blackberry Data Plan ($30). I think if you switch to the Mogul you will sorely miss your BB. I usually ask one question to every sales associate that asks about the two devices, "Does your home PC ever lock up?". Thats generally all i have to say.
Compare Phones (Phone Scoop)
While there are some misconceptions on phonescoop about PDA integrations in the blackberry (it says no, while looking at the strict definition the blackberry IS a pda) that list shows the features. For the record, I have never seen anyone get 5+ hours of talk time on a mogul, though i have used my pearl for over 4, and with GPS for several hours in a day and still had a good 30-35% battery left