So, we're considering dropping DishNetwork and just purchasing TV shows
You could supplement that (in a BIG way), by hooking up a modest antenna to your digital TV, and exploring the 20-30 local HD channels in your market. For FREE.
Darn, I forgot to suggest that!

That's a good enough excuse to get a Tivo HD, though that would be a little added monthly cost. Still, for as little as $12-13 a month you'd get a lot of OTA programming. It might be worth it if you figure out how many shows you'd be watching through iTunes are major network shows. It would lessen the replay aspect, though with Tivo to Go, you can get those recordings off the Tivo pretty easily.
Plus, OTA HD quality would beat out iTunes...