What are the differences between 2 lap timers (Qstarz BL-Q1000GT vs Qstarz BL-Q818GT) ?

BL-Q818GT is a bluetooth GPS receiver.

BL-Q1000GT is a bluetooth GPS receiver and Data logger. 

BL-818GT works with QRacing APP, after you stop the laps, session will save in QRacing App.

BL-1000GT will do all BL-Q818GT can do. Furthermore, it saves data into the MicroSD card. You can use QRacing PC Software to read and analyze the data from the MicroSD card.