I can't compare the AppleTV 4 but we do have 3 AppleTV 3s and a Roku 4 so I can compare those. We're happy with the Roku 4 and haven't had any problems with it. The UI is fast and responsive and looks nice. Stuff starts playing very quickly. Voice search seems to work well but I only tried it a few times. Honestly, I don't see myself using that feature a whole lot, and I wouldn't doubt it if Siri works better, but it's still cool.
I don't think the new AppleTV sucks at all. It seems like a nice device that has a lot of potential thanks to the app store. I chose a Roku 4 over an AppleTV 4 because of the first 3 reasons I list below although I'm not opposed to getting an AppleTV 4 as well in the future if there turns out to be tons of support for the app store.
Upsides of the Roku 4:
* Amazon Prime. I think the only way Amazon will ever bring this to the AppleTV 4 is if their Fire TV device fails and they decide to get out of that business. Sure, you can AirPlay it but I much prefer using a native app. Just like I don't expect an iTunes app on the Fire TV, I don't expect a Prime app on the AppleTV. They're very much competitors and they're both trying to own the living room right now. No Prime support was always my biggest complaint about the AppleTV 3.
* 4K support. Yes, it's true there isn't exactly a whole lot of 4K content yet so lots of people might not care about this but more will come. It's a nice feature for those of us who do have 4K TVs and don't want to use the TV's built-in apps (which suck on my TV). Supporting 4K at 60Hz is a nice bonus but just about any 4k content I'd care about would be at 24 or 30 hz so I wouldn't choose a Roku over a FireTV just for 60Hz support.
* The device doesn't try to steer you toward any particular content provider. It's agnostic.
* It has an optical audio port. I don't use it but it's a nice feature for those that need it. I certainly get why some are upset at Apple for removing it. Roku removed it from the Roku 3 and brought it back because so many people complained about it.
* Slightly cheaper than the AppleTV
* Sling TV support although I don't currently use it. I think whether or not this comes to the AppleTV will depend on how well it sells and if the market demands it.
Downsides of the Roku 4:
* It has a fan inside it but I honestly can't hear it unless I put my ear within 1 to 2 feet of it. It's certainly much quieter than any HTPC that I've used in the past. Still though, having a fan means there's a chance the device could overheat and break if/when the fan stops working. Some have complained about fan noise but I think they have defective units. I also saw a report about it getting too hot but mine doesn't get hot at all.
* The UI is not as refined as something you'd find on an Apple device but it's certainly usable.
* Searching seems to work well, however, it will most likely never be able to search your local Plex library. The Plex developers say it's because the Roku can only search cloud services and not local content. While they didn't mention other specific devices, they did say the Roku is not the only device that works like this. I wonder if the AppleTV is the same.
* Ads on the home screen. Yes, it's lame but they don't bother us.
* No 24Hz support nor is there support for bit streaming HD codecs but I didn't expect a device like this to support those things. I don't expect the AppleTV 4 to ever support these things either. I'll probably build an HTPC just for plex so I can have these things since they are important to me.
* No access to iTunes content obviously but we don't purchase movies/tv shows off iTunes because we don't want to be locked into Apple.
There are probably more pros/cons but those are the ones I noticed so far.
The big reasons to go with the AppleTV are iTunes content and the potential for the app store. We still keep the AppleTV 3 hooked up in case we want to use iTunes. If the app store winds up truly becoming some revolutionary thing that provides apps you can't get anywhere else, then we'll consider getting an AppleTV 4 as well. However, I can't help but think that any must-have app that's on the AppleTV 4 (aside from iTunes obviously) will most likely make its way to the Roku/FireTV/etc. I do, however, think the AppleTV has more opportunities for casual gaming but I have more than enough ways to satisfy my video gaming needs at the moment and iOS gaming in general never really interested me all that much.
Airplay is another reason to pick an AppleTV over a Roku but we still have an AppleTV 3 for that. Plus, some of Roku's apps do actually support the same thing. So far, I've only tried it with the YouTube app and it worked great.
I'm not going to get into the remote controls. I don't really care about the remotes that come with devices like this since I use a URC MX-890 for everything. Things like HDMI-CEC are nice and all but if you have more than one device, a good old fashioned universal remote (ideally with RF support) is still the way to go. One thing I will say about the Roku remote is that I like that it has a dedicated home button and buttons for several of its channels, all of which I have my URC remote programmed to use (you can get the codes from remotecentral.com since the included remote is RF only).
So, for me at least, the Roku 4 is the better device but that doesn't mean the Roku 4 is the better device for everybody. If you're heavily invested in the Apple ecosystem, the AppleTV will be the better device.