We were on The Diamond Princess Mexican Riviera cruise this past Nov, we had an inside room which we got very cheap (for myself, DH and DS age 15) we paid just over $500 each taxes, fees, port charges included.
Our NCL Spirit repositioning LA to Vancouver 2005 was also inside stateroom.
Friends on the Diamond had a Balcony on Baha deck.....VERY NICE!!! Room was slightly larger but LOVED the balcony! Great for pre dinner drinks or morning coffees.
I think if it is only $400-$500 more for both of you then I'd do the balcony, I would splurge. On the way down we saw alot of dolphins playing so close to the ship and to have had a balcony to see them closer would have been magical
Have a great cruise