Basically you have to decide what is most important to you. Do you want to have the widest range of content offerings(Roku), or the best compatibility between other Apple devices and software(Apple TV)?
The Roku box for example can play Hulu and Hulu Plus. The Hulu service is in very high demand and a very good content stream for the Roku. The Apple TV at this time will not play it out of the box, although it can be hacked to do so.
The biggest advantage that the Apple TV has is if you own or plan to use an iOS device in conjunction with it. For example, you can use AirPlay to stream music, photos, video, and apps to your Apple TV. This can be very handy in many situations. If you don't already have an iPhone, iPad, or the like - and don't plan on getting one anytime soon, then this shouldn't be a big draw.
The Apple TV also works flawlessly with iTunes. For example you can use iTunes Match then stream the audio to your Apple TV very easily. Roku can do some similar things with iTunes, but is does not have the same level of integration.
Overall, both devices have benefits and downsides, it just depends what you think your primary use will be and what hardware ecosystem you have. For more detailed information I would take a look at this review that compares the two.
I have an Airport Express and 2 ATV's, and they all get along fine, so it is possible. First, can you take you AE out of the equation? Does the ATV work fine (wired or wireless) without the AE?
If so, add the AE into the mix. You haven't explained what you're using it for. I can't think of a valid reason to have them in the same room, so I'm going to assume you either need to extend your wireless range to reach the ATV in a remote part of the house or you just want to hang some speakers on the AE's audio port.
In either case, this works for me. Setup the AE in the Airport Utility's "Wireless" tab (in Mac OS X) to "Extend a wireless network", and make sure to send the AE's network name to the SAME as the main wireless network. This works really well to extend you wireless network into remote parts of the house!
I guess you could be using the AE as the (only) wireless router, connected to an Ethernet-only router, and connecting the ATV to this (only) wireless network. In that case you'd select "Create a wireless network", and join the ATB to this network.
If you could provide more info about your setup, it might help with more suggestions!
Best Answer
I've downloaded Apple TV software 7.5 from https://ipsw.me/download/AppleTV3,2/12H885 and the Finder info says it was downloaded from http://updates-http.cdn-apple.com/2020/ios/061-27784-20200313-6EA1D1F6-A6E5-40CB-8C61-B9680241903C/, which apparently is an Apple server.