So I have done 6 cruises in my lifetime - can't wait to plan the next one!! I have done it all - from sharing an inner cabin with 3 other high school students, to a mini-suite with balcony for me & husband.
The nice thing about having a balcony is that you can go out & get some private time & just BE... enjoy the ocean, watch a sunset, read a book... very peaceful, esp in the warmer waters.
What do you & your husband tend to do on vacation? If you go out & see some sights, & then come back to the hotel after dinner & hang out, or even order room services & dine-in, having the Grand Suite would be nice - more room to stretch out & have all the "comforts of home". But if you tend to use the room as a place to sleep, & maybe unwind a little infrequently, then it is really not a necessity. Rooms with balconies are already a bit bigger than interior cabins, so you won't be lacking for room to move around if you take a balcony room on either line. Also, length of cruise is important. 3-5 days is a non-issue, I can do tiny interior room for a short cruise. Stretch it out to 8-10, or more days, & then yeah, I want the extra room to feel comfortable.
As for Carnival vs. Royal Caribbean, I haven't been on either (I've done Disney, Norwegian & Holland America), but which cruise I took would depend on the destinations. If Royal Caribbean has an extra stop in the same amount of time, check the amount of time spent at each location. If you prefer "seeing the sights" & they reduce the amount of time in port @ other places to squeeze in their island, then take Carnival. If you have always dreamed of lounging on the beach in paradise, the private island would be right up your alley.
In regards to cost Doris Day mentioned, keep in mind that most (if not all) cruise lines are making tipping "easier" by charging a per-diem. This way, guests don't need to run around with envelopes of cash to hand to all their favorite service staff on the last day. But it also means budgeting for a bit more $$. If you have a really outstanding cabin staff or server, of course feel free to tip them above & beyond. Years ago, we paid for internet service, & it was charged in increments of time. We got permission to give the cards with unused time to our cabin staff so that they could email/skype/etc. with families without having to pay. So showing appreciation can be done in other ways as well.
Have fun, & safe sailing! T. :)