What's the difference between a collar-activated and a microchip pet door?

Collar-activated doors (like the High Tech Pet Power Pet line) read an ultrasonic or RFID collar tag your pet wears, while microchip doors (like SureFlap) read the chip already implanted in your pet so there's no collar to lose. Reviewers note collar systems are easy to add to multiple pets, while microchip doors avoid the dangling-key problem but are typically flap-style rather than motorized.