Regarding SmartTV versus a Streaming box/stick, I'd go with the external solution unless you're prepared to swap out your TV every 3-5 years. If you get the SmartTV, you're stuck with whatever CPU, RAM, and storage that came with your SmartTV, period. If Roku, Amazon, or Apple come out with new services that require faster processing, you can bite the bullet and pay another $30-200 for a new device instead of $500-3000 for a new TV. Obviously, the bigger the TV screen, the more sense it makes to go external. At least until the TV manufactures come up with a industry-standard slot that allows the Smarts to be swapped out and keep the rest of the viewing hardware in place.
Sometimes you have to go internal, like a YouTube app that does 4K HDR that may not be supported by your external box's hardware (*cough*, Apple) but that doesn't happen too often.