Had the same decision to make 2 years ago with a baby and fairly regular trips to the UK, as the car had roof rails (rover streetwise) we got a massive roofbox (470L I think) instead 2nd hand inc thule bars for about £200. It meant I could get all the stuff in that I needed too so long as I packed well and still take 3 people (mother in law nicking free trips back to england) and the nipper.
It weighs about 15kg, which is easy enough to put onto the roof of the car and is stored in the shed with no problems (stood up at the end out of the way).
We've since gotten a mondeo estate and we still use the roofbox when making any half decent long journey's (it has integral roof bars too). On a trip from Holyhead to Manchester I only managed 49.6mpg as opposed to the normal 53 ish I get when doing 70, which I didn't think was too bad.
It mainly has the pram thrown into it and 20 gallons of vimto on the return journey, it just gives you more options and less having to make do/struggle a bit.