In answer to your question Graham, We had an outside cabin on Deck 2 of the Spirit in July of this year and if you want as smooth a voyage as possible, I would recommend that you get a cabin that is mid ships - that is in the middle of the ship itself - this is to avoid any rocking or rolling if the sea happens to be rough. We had a very rough night on our way to Livorno but unfortunately due to the extreme weather conditions, we couldn't dock there but we never felt any of this bad weather because of the position of our cabin and I totally agree with dolan, book a cabin of your choice by looking at the Deck plans of the ship taht you are booked on.
Deck 1 and 2 are exactly the same but you might like to book a particular cabin- e.g midship, near lifts etc.
The cabins are all the same on the two decks I would keep your money , It will pay for your drinks bill.
You can always be moved if you don't like the cabin once onboard.