Which one do you prefer and why?
I prefer VC-1 because the bit rate is superior to AVC assuming both are compared at L4 (Which is the highest VC-1 currently supports). For my setup VC-1 is "universally" accepted which makes it the codec of choice for me when I convert my videos.
VC-1 or AVC?
I am not sure that they are really all that different.
I once read that when Microsoft decided that they wanted to open VC-1 to broadcast the suddenly found that they didn't really invent much and that many things they thought were their IP were already patented by companies that contributed to AVC.
So as it turns out now, the patent pool for the two codecs are very similar.
The one that is preferable probably comes down to which implementation you can get your hands on
I once read that when Microsoft decided that they wanted to open VC-1 to broadcast the suddenly found that they didn't really invent much and that many things they thought were their IP were already patented by companies that contributed to AVC.
So as it turns out now, the patent pool for the two codecs are very similar.
The one that is preferable probably comes down to which implementation you can get your hands on