A bigger screen actually doesn't mean you will get more real estate...
Both of those screens have the same resolution. Things will only looking bigger on the Dell since the pixel pitch is higher.
On that note. I own a Dell 2709w and am very happy with it. I use it as an all in one type monitor for my macbook as well as for my PS3.
What I suggest is to determine what you are going to use the monitor for. If it is just for your macbook...then the ACD is the best option, and is actually a good price for what you are getting, although you can't even use it since you don't have mini displayport, so you would have to go with an older ACD. If you want something that has a lot of inputs for a game console or a dvd/blu-ray player or something, go with the Dell.