The specs are on the Antennacraft web page.
http://antennacraft.com/Antennas/AntennasHBU.html
The main functional difference is that the HD1200 is meant to receive VHF low, as well as VHF high and UHF. The HD1200 is also a longer antenna, 120" vs 85", which goes along with the capability of receiving the longer wavelengths of VHF low.
The HD1200 is also somewhat more sensitive to VHF high and UHF than the HBU33. The HD1200 and the HBU44 are nearly equivalent in these bands.
You can also look at the half-power beam width. This will tell you how focused the antenna is in the direction you point it. The HBU33 is the widest, at 56 degrees, with the HBU44 at 50 and the HD1200 at 33, all for VHF high. So a HBU33 may work a little better than a HBU44, when the transmitters are more spread out. Look at your simulation and try to match the beam width to the stations you want to receive.
For your mast, indoors you can use pretty much anything that's convenient. PVC plastic pipe is probably the cheapest. Screw it to the attic floor and roof rafters - it only has to support the weight of the antenna - no wind or other weather.