I've cruised both lines and like both for different reasons. I like the room size and food better on Carnival and the service, upscale feel to the ship and entertainment better on RCL. I don't think you can go wrong with either line.
1. By cruising during the second half of September you are still in hurricane season. While you may have clear sunny days and smooth sailing, be prepared for itineray changes and bad weather. I cruised the last week of August 2005 and was in the gulf when hurricane Katrina hit. While we had mostly smooth seas as the captian went out of his way to avoid the hurricane, instead of returning to Tampa we returned to Fort Lauderdale. The cruise line then bussed everyone back to Tampa. Make sure you buy travel insurance that will cover you in case of a hurricane.
2. Cruise prices are constantly changing. I was looking into a cruise before Christmas and by the time I called the price changed dramatically upward! If you're comfortable with the price book it. Any time I can cruise for less than $100/pp per day I consider it a bargain.
3. You should plan $10/pp per day for tips which is $140.00 per week for a 7 day cruise for 2. A 15% gratuity is automatically added for all bar drinks. As for you shopping/shore excursion budget, those vary by person. I've cruised and spent very little on both of those and then on other cruises have spent quite a bit depending on what we decided to do and whether or not I bought jewelry!! I plan $100/pp per day for tips, alcohol, shore excursions and shopping and most always bring money home.
I can't comment on the ship as I've never sailed her, but have been to all the ports and enjoyed them all.
Hope this helps and if you have any questions feel free to email me.