I have had an Apple TV (3rd Gen), a Roku 3, and a Chromecast all at the same time.
Apple TV - Great if you are invested in the Apple ecosystem. That is obvious. I fell for buying a few movies through iTunes but I don't do it anymore. I prefer blu-ray if I am going to own a movie. I can rent a film cheaper from other places too. If you like to AirPlay a lot or use your TV as a second monitor (you need newer laptops) then again the Apple TV is great. As for streaming services, it's no better than a Roku or a Chromecast.
Roku 3 - This little box is the ultimate device when it comes to streaming selection. You get it all. Netflix, Amazon, HBOGO, Hulu Plus, Plex (a big one), Crackle, Vudu, PBS, History, Flixster, Blockbuster on Demand, AE, Redbox, Fox Now, EPIX, Rdio, Spotify, and a whole bunch more. As someone who sometimes gets his movies from "other sources" and uses Plex as a media server, the Roku is great. Also, it is awesome to be able to plug headphones into the remote and watch something without bothering others in the house. There are some downsides though, such as a poor UI. Apart from the universal search (which is brilliant) I find the UI to be lacking, especially for Netflix and Hulu Plus. That's not to say its slow. It's fast and in some cases its faster than the Apple TV. I just personally think its ugly. Another downer, there is no YouTube app and I am not sure if you can cast YouTube to the Roku yet. You can cast local photos and movies to it though which is nice. Also, you can stream content off of an external hard drive attached to it.
Chromecast - This is the only device I use nowadays, and I am sure most of you on here will call me crazy for doing so and disregard my opinion on all of this. For $35, the CC is a great little device and does everything I need it to. Personally, I think just using your phone or tablet to serve up media is the future. Why should we waste time making big clunky UI's for the TV and browse through them with a remote, when we already have nice UI's right on our phones and tablets. It's so simple. I love just taking out my phone, opening Netflix, and casting a movie or episode to my TV in a second. I use Google Music, Netflix, Hulu Plus, HBO GO, Plex, and YouTube with my CC all the time. Watching YouTube is great with the CC. It plays at a higher quality than just AirPlaying a youtube video to an Apple TV, and you can queue up other media. What I really love about the CC is that IT streams the media, unlike AirPlay. I can watch something else on my device without the CC cutting out or I could shut my device down completely, and the CC will keep streaming. Because it's as simple as adding the API into your app it makes it incredibly easy for apps to add the functionality. Every week we see more and more apps adding Chromecast support. Then there is the whole other feature where you can cast a chrome tab to your TV, or your whole desktop (BETA). It's also great that it plugs right into your TV and it's not another box laying around. Love me or hate me, but I think the CC is the best device for my TV needs right now. There is one thing I will say about it though and that is I wish it had a physical remote. As an iOS user, I don't get the nice notification of what is playing with controls like on Android. I have to open the app that is casting, which can be buggy sometimes. It would be nice to just have a physical remote to quickly pause or play something.
The only thing I would be looking forward to in a new Apple TV is an app store, so that anyone could add their apps to the device. I want a Plex app and an Amazon Instant app. However, seeing as how Amazon is set to release a set top box this week I doubt that is going to happen.