Roof Rack or Cargo box?

Guys,

I'm currently in the market for a mini-van and had thought I'd just go
pick up a Thule ski rack but then started wondering about security and
such plus whatever other issues I might not be aware of regarding the
benefits and drawbacks of a rack versus a box.

The big question I would have with a box is, is it easily removable
between trips? I wouldn't want a cargo box if it's a permanent fixture
on the vehicle. Second, how the heck are you supposed to really get to
it on a van? I guess you just open a side-door and get to it that way?

Is a 6-pair rack good enough? I guess it would only make sense to
remove the skis from it if you're going to be gone any length of time.
I'd hate to have to worry about my skis if we stop on the road for a
lunch and come out to find no skis.

Much of the main reason for looking into the van is to be able to keep
the skis out of the way on the roof. If this isn't very feasible,
reliable or safe, I'll have to re-think the whole thing.