The most notable advantage of the newer monitors is the improvement in terms of brightness level in stereo 3D mode and further reduced ghosting as compared to older models. The advantage of getting a model with built-in IR emitter and bundled glasses is that you get to have HDMI 1.4 3D support available and it usually comes cheaper than getting a kit separately.
If you're looking for the best 3D monitor that will also play great in 2D, then you should choose between the BenQ XL2420T and the ASUS VG278H. Both incorporate nVidia's 3D Vision 2 with 3D LightBoost which greatly improves the overall 3D experience.
While the BenQ doesn't disappoint as far as specifications with a 2ms response time, 120Hz refresh rate, low input lag, and a great picture, The XL2420T comes with a lot of other features built-in to this monitor specifically for gamers including FPS Mode, RTS Mode, Intuitive OSD, S. Switch;Height Adjustment, and Game Mode Loader.
If you just want this monitor for the 2D technology, then you might want to consider its predecessor, XL2410T, which is significantly cheaper and has many of the same features as the XL2420T, but has nVidia 3D Vision rather than 3D Vision 2.