I'd look at how much stuff you want to carry and then look at what the dynamic load you're allowed to carry on your roof is. It's an important factor. Our Navaras can only carry 95kg up top while on the move, but can support about 250Kg when stopped.
If you can organise the heavy stuff into the car and only have bulky light stuff up the top - and you can keep it dry and clean, because sure as eggs it'll bloody pour down when you go out without that sort of protection - then do it.
The other side of the coin is how handy a trailer can be, as has been pointed out. It needs to be stored, it needs to be remembered when overtaking or parking and it will use more fuel than loading up the roof, but it will carry a lot more too.
We used to take a loaded 6x8 box trailer behind our Magna wagon, which was filled to the roof in the back. The trailer would have a tarp placed inside underneath everything, then another over the top of the ropes tying everything down with a trailer net holding it all in place.
It depends on what you want to do with the vehicle too. If you want to do desert sand driving, forget the trailer it's too much dead weight. For formed tracks or roads, the trailer won't be too much of a bother.