Roku has a Giant Bomb app, Apple TV does not. Apple TV3 cannot be jailbroken to play your local media via XBMC, you'd need to go back to the older ATV2 if you can find one to use XBMC/Plex, etc. Even then you're limited to 720p. If you're not heavily invested in the Apple ecosystem I'd go Roku.
Oh, there is an unofficial Giant Bomb app and an official Twitch app on the iPad that you could Airplay to the Apple TV in the worst case scenario.
Edit: Missed the fact you have a Macbook. Suppose you can Airplay most anything then. My vote is still Roku though!
I just bought a Roku 3 last Saturday finding myself one month away from my xbox live gold sub coming to an end and not really wanting to use the brand new PS4 to stream Netflix all the time. I'm beyond happy with my purchase. The interface is snappy, set up was a breeze, the search function is amazing that it gives you results found in all of your roku apps and most importantly unprofessional Fridays on my tv without having to hook up my MacBook. Also the headphone jack on the remote is a nice extra feature. Go Roku.
I have had a Roku for a few years and wouldn't recommend against it. Sometimes the UI gets a little laggy and shits out if you have it on for too long (I'm talking weeks if not months here), but it restarts in an acceptable amount of time.
Personally I went with Roku for a few reasons:
1. Better "channel' variety.
2. If I feel like it I can jailbreak it for more variety.
3. I was/am not heavily invested in Apple products.
Personally I like the Apple TV just because I like the interface and the limited selection is fine for me (I use iTunes a lot, and for the few times I need to use Amazon my TV can handle that), but for your purposes it sounds like the Roku is what you're looking for.
I'm not so sure how compatible Apple TV is with a lot of video formats like MKV. I would go with Roku. Airplay from my experience is not the most reliable method of pushing content to your TV. (I think the Tested.com guys said the same thing too)
I would go for the Roku. It has a bunch of app like Giant Bomb, Plex, Netflix, HBO Go, Hulu, Spotify, Amazon Instant, Amazon MP3 streaming, and a bunch more. Seems like you are looking for a box to stream video files. I have been using Plex to stream my DVD collection and it's fantastic. I've used a lot of streaming software like TVersity, PS Media Server, Windows DLNA client and I always had problems with them connecting. Plex is the first program that has been problem free for me. Has transcoding so file compatibility should not be a problem and it auto tags you files with movie posters, directors, actors and stuff. One cool feature it has is when you select a TV show, the theme song will play while you browse the episodes. Plex also is a DLNA client so it will work on PS3 and 360 but the apps for Plex is a much better experience.
The Giant Bomb App is awesome too. You can sign it to get HD video and subscriber only content but you can't watch any live streams on it. I think you could watch GB live using the Justin TV app but that app is not good.
My two negatives for the Roku is the lack of a YouTube app. Luckily my Panasonic has a YouTube app built in so I can use that, but no YouTube on Roku really sucks. Also if you are a Comcast subscriber, you can't use HBO GO. You are the worst, comcast. If those 2 problems were fixed, I would probably buy another one for my other TV.
I don't know much about the WD TV except that it has no Plex app.