This thread topic must come up about once a week or more. If you like Roku, I would stick with that. I personally think the ATV is better, and I get reminded of that every time I use my Roku or Firestick, but if you haven't used it yet, and are happy with the Roku, might as well save money and stick with it.
I posted this or something similar on many other threads, basic pros and cons of different offerings:
Pros to use a Smart TV, Roku, Fire Stick etc. over an ATV:
Lower cost, some smart TVs cost less than just an ATV
Built-in and convenient
Buttons on TV's remote to activate apps
Cleaner set up, less wires and devices to hook up
Easier and quicker set up for video settings
Cons for Smart TV, Roku, Fire Stick etc.:
Lack of long-term support, Apps could stop working or be missing new features over time
Slower than ATV, even much older ATVs
UI isn't always great
Lack of privacy, many Smart TV OS sell user data without given an chance to opt-out
Pros to having an ATV:
Long term support
In Apple's ecosystem, some apps are available across different Apple devices
Homekit, iCloud, Remote app
Better UI, although this is subjective
Faster HW
Gaming* Due to dropping gaming developer support for tvOS, this is starting to go away as a "pro"
Privacy, Apple is pretty good here
Cons
Cost is probably the biggest
Apple doesn't seem to care about tvOS nor the ATV, although this is just my opinion
The remote, the Siri Remote is not widely liked, but the newer remote cannot be used on many gaming apps
Not as clean set up
Higher video settings require a more expensive HDMI cable
Setting up for best picture quality can be a hassle, imo
Now that AirPlay, ATV+, Apple's TV app, and HomeKit are being offered on some non-Apple devices, the benefits of the Apple TV are dropping.
The Apple TV is basically an overpowered, and overpriced streaming box. So much untapped potential, but pretty much back to being Apple's hobby.
Apple doesn't seem to care much about the Apple TV, and I wouldn't be surprised if Apple discontinues it over the next few years.