There IS a practical difference between CDMA and GSM; which is where the user account information is stored.
In most CDMA phones, the information is stored within the phone. If you change phones, you need to get the new phone activated by your cellular company.
GSM stores your information, as well as phone numbers on a "smart" card, which means that you can simply remove the card and put it into another compatible phone and have account and phone numbers transferred to the other phone.
Addiitionally, if you are travelling to Europe, you can bring your GSM card and put it into a European frequency GSM phone and use it there. Alternately, you can get tri-band GSM phones that work in the US and Europe.
TTFN