Nokiaman is correct. Assuming you purchase an unlocked 6682 with the 850 bandwith the rf will be the same if not better since you at times may pick up other carriers signals in dead spots. The 6682 is the 2006 car while the 6620 is the 2005. Both are going to work great but the 6682 will have some upgrades such as:
Better stereo speakers
Better color display
Quicker internet with faster Edge
Larger screen
Slightly larger in length and weight, but feels better than the 6620
Hot Swappable Memory, which is great for switching out different memory cards rather than having to turn off the phone and take out the battery.
Only downside- Less memory than the 6620, but unless you like to listen to mp3s, have agile messangers open, opera open and play videos games on your phone all at the same time you will be just fine. I highly recommend the 6682
