M.G.
We have both. A soft one with a zipper and also a hard top one.
We often use both together. We need a bigger vehicle!
The hard top is easier to pack. And it's more secure. It has a lock. It would be harder to break into - if someone was going to try. It's waterproof. It holds a ton. And we don't have the biggest one. So when we camp, we can fit a lot up there - and it's great for tossing the sleeping bags, and duffle bags and bits and pieces like beach toys and chairs.
The soft one is just not as good when it comes to rain - and you have to strap it all down.
The hard top one is screwed into our car's roof somehow on our rack. My husband handles that but easy enough to get on/off.
I think either would handle a couple of suitcases. We started with the soft then upgraded but kept the soft for stuff we didn't worry about getting wet.
Hope that helps :)