Roof rack.
Disadvantages:
a) Precarious
Pain in the arse to lift bikes onto it and secure them
c) Very fiddly, bars have to be slid onto rails and little tiny plates have to be bolted on in the correct position, have to be extra careful not to round bolts, etc.
d) Extra pain in the arse (to such an extent that you'll be lucky to do it) to put bikes on when it's dark
e) Security - bike's highly visible
Advantages:
None really
Rear carrier.
Disadvantages:
a) Wobbly
When you put it on, it wobbles about, even before bikes are on it
c) Can scratch the car
d) It has a tendency to wobble
e) Can't open the boot when it's on
f) Did I mention that it's wobbly?
Advantages: none really
Disadvantages:
a) Precarious
Pain in the arse to lift bikes onto it and secure them
c) Very fiddly, bars have to be slid onto rails and little tiny plates have to be bolted on in the correct position, have to be extra careful not to round bolts, etc.
d) Extra pain in the arse (to such an extent that you'll be lucky to do it) to put bikes on when it's dark
e) Security - bike's highly visible
Advantages:
None really
Rear carrier.
Disadvantages:
a) Wobbly
When you put it on, it wobbles about, even before bikes are on it
c) Can scratch the car
d) It has a tendency to wobble
e) Can't open the boot when it's on
f) Did I mention that it's wobbly?
Advantages: none really