I have had both several times, and to me it is not worth the extra cash, except to be able to say you have done it.
You will have more room, and with the just the two of you, much more room. You will be able to board the ship in one of the first groups. The balcony might be bigger, but other than that, not much else.
We had a suite on the Spirit, so if that cruise is only 5 or 7 days, why not book the suite on that ship, instead of your B2B.
Have a great time,