721 vs. TiVo Series 2

I know this is somewhat a difficult comparison to make since the 721 is in limited release and the series 2 TiVo isn't even out yet, but with that said what are the advantages/disadvantages of each? I'm a current Dish subscriber and happy owner of a 501 and am looking to upgrade to something with dual tuners. What does the 721 have that the Series 2 won't and visa versa?

Thanks!