You know, you really can't compare the two. When you're using a BlackBerry you'll wonder how come so many people torture themselves with a SideKick.
The biggest Thumbs-Up, to me, is that I manage my data. I don't rely on incompetant third-parties that have done nothing but demonstrate that they just don't get a fsck what happens to my data.
I don't use a BlackBerry as a mobile phone - I use it as a tether to everyone I know, as I'm sure you and your fellow SKers do. To not have control over your own data is an incredible limitation.
You'll be able to change from two devices to one, and probably end up paying less, for better more reliable service. Save that right now we have a nationwide service outage