As OpenGL version a suggest 3.3+ because almoste all modern graphic card support it and is very similar to OpenGL 4.0.Second this.
Although OpenGL 2.1 is the "equivalent" of DX9, there is no good reason not to use OpenGL 3.x for you (but there are good reasons against using OpenGL 2.x).
Since you come from DX9, you will have to learn a new API in any case, so best learn the "correct" version right away. Otherwise, you'll have to spend extra time at some point int the future on unlearning and re-learning. Going from 3.x to 4.x is a breeze, but there is a huge paradigm cut between 2.x and 3.x.