How much do you want a cabin in a specific location? Do you need to be close to an elevator?
If there isn't a major need to be in a specific location I would book the guarantee. You will not receive a cabin that is in a category lower than that and you do have a "chance" of an upgrade but don't think it's a sure thing.
I've had a couple of upgrades when doing this but I have never ended up with a cabin I did not like.