That all depends on your preference. Everyone is different. Go to your carrier store and see what they provide and handle the devices to get a feel.
Go to this site to compare the different BB's available. BlackBerry - Compare BlackBerry Smartphones and PDAs at BlackBerry.com
NOTE - not all BB's work everywhere or with every carrier. So you must know which BB's your carrier offers.
And most important, remember that BB's need a Blackberry Data Plan
to operate properly. This is not the same as a data plan for other phones. Ask your carrier about this.