DirecTV will be adding Spgfld locals very soon. They recently announced this in a press release. At the latest, this should be sometime in the 1Q '07. That would give you a pretty decent backup source for network HD programming, if you have a problem w/ OTA.
Here are the HD offerings from:E*D*
E* (Echostar Dish Network) downrezzes some 1080i channels to something less than 1920x1080 - not sure of the exact resolution. D* (DirecTV) downrezzes all 1920x1080i channels to 1220x1080i. Neither service downrezzes 720p programming. If your display cannot resolve full 1920x1080i, you won't see any difference in PQ on D* or E* vs OTA. If it can, you will notice some difference on a big screen. OTA will nearly always give you the best PQ, and IMO, is the best way to watch network HDTV. It also eliminates potential problems that may happen with a middleman provider like cable or sat.
On cable, if you have a TV with a QAM digital cable tuner, you should be able to connect it directly to the cable without a box, and get all your local channels in HD, for the cost of expanded basic analog cable package. I have no idea if mediacom offers any other HD programming besides locals.
For OTA antenna choices, it depends on how far you are from the transmitters, terrain and stuff your house is constructed of. You may only need a simple indoor or attic antenna, or small outdoor antenna. If you want to try an indoor antenna, start simple and buy an antenna locally from a place that takes returns.