Unless you are carting a fully built rigid travel cot around then it takes up no space at all - my North Face Hammerhead when fully stuffed with kit and water is about the size of the travel cot we have. I'd suggest seeing what you are carrying and remove the clutter.
Travel cot - this packs down small (and is the one we use but there are others of similar size and design) - 2 minutes to set up and no probs with it so far.
Pram, we have a big unit - Mamas & Papas Skate - but it folds down nice and flat and if I remove the rear wheels (QR-type affair), it sits no probs with a load of kit on top of it.
I can easily get all kit for a week's holiday in Centre PArcs in a Fiat Bravo with 2 bikes on the roof, the baggage for me and the missus plus all the kit for our daughter (including a myraid of toys that the good lady insists we take and never gets looked at!). It's tight in the boot but it all fits and is no probs. However, it does come down to size of goods once packed away...so if the cot and/or pram are lumpy units that don't fold away then you have problems.
I wouldn't think of getting rid of the 407, it's a large car, not trying to sound cheeky but I'd suggest you have a look at what you are needing to take and how it is packed...there is probably stuff you don't actually need (like 2 packs of nappies instead of 5 packs, etc.) and the way the car is packed might also be improved - foot space beneath the kids seat can be packed with stuff, as long as there is enough space for the kid's legs to move around then it is all good...use stacking boxes (collapsable units from the likes of Tesco or Homebase) - these can sit in the passenger seats and not roll around but it does help conserve space and give fixed dimensions to things needing packed.
Roofbox would be my suggestion if it needs to be one of the 2 options you mentioned - it's going to be cheaper and unless you have issues with the 407, it's a fine car to keep.
