Hi Turtlebed,
You’ve probably read it already but just in case I would make sure you’ve read the mattress shopping tutorial here which may save you some frustration.
There may also be other options in your area that would also be worth considering (the forum search link in my last reply includes two posts that have lists for the Sacramento and San Francisco areas which would probably include the better options that are close to you if you are near one of The Natural Mattress Store’s locations).
A zip cover can be either snug or loose depending on the thickness of the materials inside them and on the thickness of the cover but most component mattresses have tight fitting covers.
If a mattress has component layers then there would be a significant advantage to having a zip cover so you can access the layers and rearrange or exchange them either initially or down the road. There is more about the pros and cons of a mattress with loose layers and a zip cover and a mattress with glued layers with a cover that is sewn shut in post #2 here and the post it links to.
Phoenix