car top vs. rear bench cargo carriers

Hi! I've got a Honda Odyssey and need more space for our luggage for
an upcoming road trip. I've been trying to decide between the car top
vs. rear bench type carriers. I don't have one yet, but I'm planning
to add a hitch to my van for a bike rack anyway. So, that part
wouldn't be a problem for the rear basket (? not sure what it's called
?).

I don't want to spend a ton of $ b/c we really only need it if there's
more than our own family in the car. We just happen to have two extra
riders w/ us for our upcoming trip. I guess it would also be good to
have if we ever need extra space just b/c of bulky cargo in the van.
Also, we only have the two side luggage rails on top of our van and I
don't want to have to purchase any crossbars....

Anyway, I would like to know if the car top carriers really put a ton
of drag on the vehicle due to the wind. If so, how much mileage
should I possibly lose if I take this route? Also, if I do go w/ the
car top carrier, I would more than likely buy a soft carrier due to
the expense and required crossbars w/ the hard shell ones. If I
purchase a soft carrier that is not waterproof, but spray it down good
w/ Scotch Guard are some other repellent, do you think that would
render it as "waterproof"? If I put atleast 4 fully packed suitcases
in the car top carrier, should that be too much weight on the roof of
my Odyssey? BTW, is there somewhere that I can find out what the roof
weight limit would be w/o any kind of crossbars (just sitting on the
roof)?

Regarding the rear bench type, I shouldn't get any drag w/ this type,
correct? How easy should it be to load this type (w/o losing the load
in transit) vs. the car top type? Also, if I get the rear type and
obviously attach it to the hitch, should I have any trouble raising my
hatchback on the Odyssey to get access into the back?

Sorry for the million questions, but any and all feedback is really
appreciated. I'm just having a really tough time figuring out what I
should do here!

Thanks!
Jennifer