If you want a Balcony and only will be happy with a Balcony, then get the Balcony Guarantee. You may get upgraded to a better Balcony this way (a higher category price than you pay with the Guarantee price) but it is doubtful that you will get an upgrade from the Interior to a Balcony. You may get upgraded from the interior to an oceanview however.
But to answer your question, if you have a particular cabin you think is a good location or is where you want it, then you are better off just paying for it and picking it yourself. The guarantee cabin only works if you are on a un-filled ship. Rarely to they have "the good cabins" left over to be upgraded into. The idea is to allow you to pay a lower price for your category and have the chance to be upgraded into a slightly higher priced cabin.
If you book into a balcony guarantee you cannot be put into a category lower than a balcony. With the guarantee you might get upgraded to a balcony on a higher deck than the one you paid for. Therefore you would be getting an upgrade.
Wow CF.. you said it in one sentence... took me a couple.
Your welcome JuniorCruiser. CF had a more to the point answer I guess. Guarantee cabins are least expensive cabin price for that category. IF you want to have the chance at an upgrade do the Guarantee, BUT make sure you book a Balcony guarantee (at the minimum category you will be have with) so that at the minimum you get your balcony. IF you had a certain Balcony cabin in mind that you want, buy it by cabin number to make sure you get it. You will pay more for the cabin that way, but you'll get what you want.
I can actually see that Guarantee Cabins is not avaiable for all sailing dates.
Why? Is it only the cruises that will sail shortly, or is Guarantee also avaiable for cruises that is reservated for more than a year before the sailing date?