BMW test drive a R850 k cash purchase car

So I'm driving a Mercedes Benz, at the time they allowed me to test drive from the Friday morning and return the car Monday at lunch time. So I enquired if I can test drive a new 3 series BMW to decide if I will buy a new Merc , Cash at R850k or move back to BMW. BMW only allow a 20 to 30 min test drive. So how would you make the correct choice by spending nearly one million rand on a car you would probably drive for 3 to 4 years in 20 min. It's ridiculous