RFID:
relative position
RFIDs are passive, so users have to be active, meaning a RFID reader has to be quite close to IDs. Therefore, users have to be conscious of tagged object in advance.
association between objects and ID is predescribed by somebody who attached the tag.

GPS:
absolute position
need a mechanism for mapping GPS value(numerial value) and cognitive location(a park/a campus/a home). we don't live in numerical world. this is a very strict problem in dynamic situation where objects move around and cognitive meaning of places changes.

these are not only technical issue but important elements that decides whole interface and user's experience.