The ATSC tuner in both models is a full HD tuner that can receive 1080p signals. In both cases, the TV signal is scaled to the physical display characteristics of the monitors => in the 220HD's case that means it's scaled to the smaller screen with a native 1680 x 1050 resolution. It will look FINE in both cases :-) A 22" monitor simply has a native resolution smaller than the full HD resolution --> I watch HD material on my 22" frequently, and it looks perfect ... just as good as on my 60" HDTV (although clearly I watch it from MUCH closer than you plan to --- I think a 22" display is much too small to watch from 11 feet away ... I sit closer than that to my 60" display, which is over 7 times as large !!).
As for the "1200" question => note that both the 22" [at 1680 x 1050] and the 24" [at 1920 x 1200] have a 16:10 aspect ratio. This is true for all widescreen monitors. Widescreen televisions, on the other hand, use a 16:9 aspect ratio. That's the only difference. So if an HD program is displayed correctly on your monitor, there will be a small black space on the top and bottom to compensate for the incorrect aspect ratio -- but the program itself will look perfect. Some viewers will slightly stretch the material to fill the screen ... and most folks don't notice that [I always prefer to use the correct aspect ratio].
Bottom line: Either of the monitors you're considering will work just fine ... the image quality will be comparable on both; but clearly the 24" will be somewhat larger (19% larger), and will have a 1:1 match to the HD source, so it's easier to scale correctly. But you needn't be concerned about the 22" model, if that's all you want to spend.