I'm sorry, but they are different markets. You're literally asking, "Should I get the Kia, or the BMW?"
The Defy is a low-end, cheaper phone, but meant to be semi-rugged. The Atrix is currently the most powerful phone on the market, and costs more as a result. While they both have their advantages, aside from cost and durability, the Defy offers no real benefit over the Atrix.
Get the Defy if you need a semi-rugged phone, get the Atrix if you want performance.