I have tried a variety of bike carriers. These are some pitfalls:
I have a Rola racks and the Rola two-piece bike attachment. My idea compromise when I can't put the bike in the boot. Make sure the bike wheelbase will fit with a Rola two-piece - otherwise the Rola one-piece attachment will be needed. My bike just fits, but I like the quick-release fitment of the front wheel.
Downsides: the insect grime on a new bike! (not a huge issue) And last car was a tall 4WD - never realised how heavy a MTB can be when trying to lift it up on tippytoes. PITA, ended up scratching roof. I now have a BH road bike, so maybe that wouldn't be so bad on a 4WD, but my replacement, a W124 sedan, is much, much easier for putting bikes on the roof anyway.
As for the boot-attachment racks , watch out if you have a sedan or other type of vehicle with large rear panel sections where the rack will sit. My sedan's boot top bowed slightly when I tensioned down the rack, even though the rack has large protective pads, it is a lot of pressure on the panel. I try to have the pads close to the edge of the boot to aviod this, though the shape of rack it should be in the middle of the boot.
I have a Repco towball-mount 3-bike attachment for the wagon, I bought it in a hurry. I had to file down the sharp edges of the mount base as I could see it slicing a hole in somebody - terrible workmanship. It also does not pivot down like some others, so have to take it off to get anything out of the back. If you do get a towball, this would probably be ideal (ie NOT a Repco fixed type, but I'm sure I've seen better-quality ones, and that you can have with a pivot-down feature if you have a wagon) - except you may need an additonal no. plate made up if the original is obscured.
Lastly, if you are fussy about yr bike and don't want to scratch it, make sure you put gaffer tape or similar on the bar areas where it is clamped or strapped to the carrier (not a problem for the roof-mount, obviously).
Just wish I knew all these things before wasting money on too many bike carriers...