If you aren't proficient at scanner-ese, I always recommend to purchase from a dealer that specializes in scanners. They can provide additional scanner-specific services that many people aren't comfortable doing themselves (no matter how easy we try to make them) like updating firmware, installing upgrades, updating the database, and programming for a specific area.
If you are proficient, then it comes down to who you feel will provide you with the best level of service based on your own needs. Could be a scanner specialty store, could be an on-line etailer who's only service is an easy return.