I would advise to go with the latest Master Atlas to get the more "up-to-date" information. Then, find one area that has been detailed (Haalkitaine and Rhakhaan, Jaiman, Sel-Kai, Emer NW or NE - keep SE for later, or even Xa-ar), buy that, and jump in.
My heart will always say to go for the truly old ones (Cloudlords of Tanara and Iron Wind), but it's a bit more difficult to tie them back into the current Shadow World and they have much less in terms of historical background - they are more situational.
The MA3 is better in other aspects that are not immediately necessary.
From the above, get Sel-Kai if you want to go for urban adventures in a true venice-like metropolis, get Haalkitaine if you'd rather have renaissance court intrigues, and pick any of the others if you want more traditional overland adventures. The area that is provided in MA4, Bay of Izar, is not half-bad either, with a nice cultural mix (but less detailed than a full-blown campaign module, of course).
And then... well, it's not called a rabbit hole for nothing