I have both a Now TV and a Roku stick +.
The Roku stick has an "advanced wireless receiver" my Now TV stick couldn't pick the wireless signal up from my router in the other room so the Roku stick was the best option, apart from buying a wireless range extender.
I suggest getting the Roku 4k stick if you want to watch Prime etc all on one stick, you can also auto set the refresh rate so you get 50/60hz so everything will play in correct refresh rate for smoother motion. If you just want now tv and netflix but without prime video than the now tv 4k box is good with a much better picture quality than the now tv stick. You also have the option to hard wire it with enthernet. The now tv 4k box also has the auto refresh rate option for 50/60hz but the stick you will be stuck on 50hz only so netflix if you use it will not play in 60hz like a lot of content is in on there.
One huge thing to take into consideration is the ability to mirror/cast from your android phone or windows PC to your device. It is not currently possible with the Now tv stick unless you have an iPhone. If this isn't a big issue for you then the Now tv stick would be fine. It would be a very nice update to receive but I believe its due to some kind of licensing issue around live sports that stopped them adding this feature. This is just a heads up as I wasn't aware of this when making my initial purchase, although the included 1month entertainment/cinema pass makes it a bargain still for £20.
I would just add, don't forget the chromecast, that has NowTV, Netflix and Amazon Prime Video and provides better digital audio out options than the Now TV stick (and possibly some Roku models) and allows mirroring. It does not have the 30 minute live buffer of the Now TV stick though.
Whilst I would generally agree that Roku is by far the better and smoother performing stick compared to Now TV. One thing and to me the only thing Now TV has over Roku is that BT Sport is available on Now TV but not Roku.
That being said, it doesn’t come close to replacing Amazon and the poor speed performance as well as the easily overheating of the Now TV stick.