I'd take a bit of time to think, and work out how much you'd be using it, and do a few sums in your head before making the decision.
I have a simple carrier that just hooks on to the car -
this sort of thing - that's exactly the kind of thing that has trailing straps and potentially bad positioning that can obscure the number plate alluded to ^above, but needn't be a problem if you are aware of said issues. Bit of a faff to put it on, but not overly onerous. They have the added advantage of being the cheapest option available, though you're always a bit worried that the whole thing's going to fall off at any moment, and of course once it's up and on, you cant open the boot. Tbf this has never been a problem for me, and i've driven bikes up to the Lake District and Wales like this.
Roof rack mounts are also worth considering, though they also have down sides - fiddly as f**k for one thing, the roof rack is an expense in more ways than one for another, and you have to remember it's there when going under low things, a thing quite easily forgotten - my brother carries his bikes on the roof, but then he is quite tall...
Tow bar mounts are unquestionably the best option - but they have quite a price tag -
- Think £300+ for the tow bar + installation
- Probably the best part of £200 if not more for the carrier itself
- Insurance implications that i would never have thought of - good shout @proactive
Would you be using it so often that quite a significant outlay would be worth it to reduce the hassle?
I'd be tempted to start with a rear mounted cheapish option to start with, see how much you use it, get pissed off with fiddling about with it, before splashing that kind of moolah on it.