It's funny you mention the 2 receivers. I live down the road from a local store that only has those 2 HD boxes. The rest of the IRDs are SD (standard definition). The owner claimed that they're identical but I doubt it.
The Viewsat claims to do DVB-S2, the Sonicview doesn't. If I had to choose between the two, it would be the Viewsat. Just my opinion.
DVB-S2 is a new generation of modulation that is starting to show up more often that not.