We've used both before and both have their issues. To determine if you can have the back carrier, look under your back bumper. If you can have a hitch carrier, there will be a black square hole there. If there is nothing, you can't use a back carrier in your van's current shape. Now to the issues with both of these:
-We always had the collapsible versions of rooftop carriers so you either had to load the thing on the ground and then have it all up to the roof at once or attach the thing to the roof and then attempt to organize it while you are still on the ground. This type would not be my first choice.
-We got a hitch carrier more recently and love that. You can load it without a lot of trouble. The downside is the weather, While our rooftop carrier was weatherproof, our deck is not. So you can load it easily, but you really have to put a tarp over it if you have anything that can't get wet(or muddy, dirty, dusty, etc.). We did end up getting two plastic bins that fit across the deck perfectly so we don't really need the tarp all the time. Tying the whole mess down can be a chore so it one that is best done once daily. Another downside I can think of is that you can't open the back of your car unless you unload the deck so forget about packing a cooler or something (we found this out the hard way-we're really sorry to all the people at a rest stop in Louisiana for taking two parking spaces so we could remove our cooler from the trunk). Just writing this makes me so happy we upgraded to our 12 passenger van with our crew.