Ashdown... it has a really nice location, right next to the campus center, on the Charles River. If you want to figure out how far things are from the Econ dept, check out http://whereis.mit.edu/. The Econ dept and classes are in E51-52. It's a pretty social dorm, they have a lot of common space and organize events regularly. There's also a pub in the basement (The Thirsty Ear).
Cons of Ashdown... it's a pretty old building, so expect some cracked tiles, older carpet, musty smell, and dust. It doesn't have the nice facilities the newer dorms have, most apartments do not have their own kitchen, so you need to share a floor kitchen, most people get assigned to doubles the first year they live there, they have mice problems sometimes. It's going to be closed down at the end of the year and renovated into an undergrad dorm.
I hope this helps, argyecon!