In the RAM/storage equation, if money is a tight factor, I personally tend to prioritize RAM. It's pretty easy and fast (and always getting cheaper) to plug in more storage drives as needed over the lifespan of the machine -- but sadly the days of expanding RAM as needed in a Mac are basically over.*
* Unless you have an iMac 27" with user-accessible expansion slots, which unless I'm mistaken is the only RAM-upgradable machine left besides the "trash can" Mac Pro and (maybe) whatever its successor is.