I have both installed in my system. I've had the Roku since the beginning going 5 years now and used it to just launch apps (Netflix, Hulu, Prime, etc...) I wanted directly.
I added the Nvidia shield around 8 months ago to get channels from oversea from basically every country with the Apollo Group TV app, they also have a bunch of other channels along with newer movies popping up all the time with pretty good quality most of the time. My dad likes to come over to watch soccer games not included in his tv package he has at home.
The response time with C4 to Nvidia shield honestly surprised me at first with how fluid it is so that was a plus for me. I'm sure I could set up through Nvidia shield to launch the other main apps as well but haven't bothered to do it. If it could do that as well then Id choose the shield as I'm pretty sure Apollo tv doesn't run on Roku.