Here are my 2 cents:
Starting from the RWD theme
- (+) you already have a (somehow) responsive theme so you may get things for free
- (+) you may not need to change a lot of sections once you change the css.
- (+) you may benefit from future upgrades to the theme.
- (-) you don't have experience with it so it may increase a bit your development time (one time only).
- (-) not all magento features are covered in the RWD theme (for some strange reason). you might end up fixing some things that should be working by default (but these are mainly css issues, nothing big).
- (-) some extensions for older version may not be compatible with the RWD theme even if they work as intended. (again, no big deal)
Starting from the base theme
- (+) You have the experience.
- (+) It properly covers all features magento has.
- (+) You might be able to use some extensions that are not officially compatible with 1.9.
- (-) I doubt this will be upgraded in the next releases (but I may be wrong).
- (-) you will have to make it responsive on your own.
Starting from scratch (no theme at all)
- (-) your development time will increase significantly.
- (-) You will have to reinvent the wheel.
- (-) For sure you will miss something.
- (+) you have 100% control on what you display.
- (+) you get to brag to your friends that you built a magento theme from scratch.
What would I do?
This being said, I would start from the RWD theme, mainly because I don't know how to make things responsive on my own.