neoworld, the general advice you are going to get from most is to look first at the carrier that offers the best coverage in the areas where you live and work. From there you will most likely be comparing the models offered by that carrier and picking from a business phone form factor versus the PDA form factor. As far as the subtle differences like keyboard, battery life, etc., there's a good discussion addressing that on another thread that is active right now and no need to repeat. As far as plans, you've probably figured out you will have some sort of voice plan with a blackberry data plan add on. If you have a choice of carriers, I think you'll be paying close attention to the data cost. I'll let others address plans and costs.
By the way, welcome to the group!