I've got an Apple TV (2nd gen) and a Roku 2 XS. Apple TV is nice if you want to stream video you own from your PC to your TV, or to stream video from your iPhone or iPad to your TV (only some of which is supported). It has Netflix and Youtube support.
The Roku supports many more services. Netflix, Hulu Plus, Amazon Instant Video (free for Amazon Prime members), and tons of channels for sites like Revision3, Giant Bomb, etc.
My wife and I could probably get by just fine by paying $16 a month for Netflix and Hulu and the $80 a year or so for Amazon Prime, but it all depends on your viewing habits and desires. But, like others have said, I call to cancel my Directv every year or so and they drop the price and give me things like free Showtime and NFL Sunday Ticket, so I never manage to actually cancel it.