Not only is it generally cheaper to book it yourself, you get to customize where you go & stay. The packages will all put you in a hotel that 'matches' your cruising category. If you want to stay cheaper (like All Stars) or more expensive (Grand Floridian) they cannot accomodate you. Nor can they change the number of days of the land portion (if you want 5 days instead of 4).
That being said, our first trip to WDW was a 4day Land/3day Sea package, because we had *no* idea what we were getting into, and I have no regrets. Anyway, check out the boards & I'm sure you'll have more than enough info to figure it out.
Sarangel