Besides the price, the devices are quite similar except there is no rear-facing camera or HDMI port on the A200. The A200 also has a bit of a different shape from most of Acer's other tablets, and its rounded corners are not quite as distinctive as the A500's slightly tapered ones. Additionally, the A200 trades in the A500's aluminum back cover for a plastic one. After that, there are few differences between the two. Both have 1280x800p resolution, a full gigabyte of memory, microSD card slot, front-facing camera for video chatting, Tegra 2 dual-core processor and full-size USB ports. Furthermore, both should get the Android 4.0 update eventually. There's really no telling when that might be, unfortunately, because neither Acer nor Google have given any hints as to which devices will get it when. The A200 does have a slightly different software overlay than the A500, but its not too distracting if you prefer the pure Google Android underneath.
Since both have Android apps, the real difference is the HDMI port and the camera. There are few who probably care about the camera anyway, but if you like to hook up your tablet to an HDTV, you'll have to skip the A200. It that's not an issue, go ahead and save yourself $50 and get the A200. Of course, you should try them both out first if you can because some people really prefer the aluminum feel over the plastic. Tell us in the comments if you're holding out for the quad-core A700 that was announced in January at CES.