Should I have a spare cassette or a spare wheel?
It's a personal choice. If you're going to run the same tires, then just go with a spare cassette. It's pretty easy to swap them out, so I don't see that as a big deal. However, If you're going to have different tires and a different cassette, then I'd probably opt for the different wheelset. If you need a more durable wheelset for touring, you might also consider a separate wheelset.  

I tend to have a road and off-road set of tires for my cross bike. Since it's a lot more work to switch the tires each time I want to ride different types of terrain (and want a slightly larger range on the off road set), I prefer to have a separate wheelset.

It only takes a minute or two to swap a cassette, and also gives you time to get the other one good and clean while it's off the bike.