I don't know about other smart TVs, but a Samsung smart TV is leaps and bounds ahead of a Roku. Plex and Netflix run flawlessly for the most part. Roku is slower, and its remote has like 3 buttons. The interface also looks much nicer on the Samsung than the Roku. I have had the Roku 1, Roku 2, and several Samsung smart TVs. I also have an HTPC.
While the HTPC is slightly faster than the Samsung smart TVs, it cost a whole lot more. Can you even buy a decent sized TV without it being a smart TV? Why would anyone not buy a smart TV? While an HTPC is nice, I still haven't found there to be any good interfaces for them. Samsung's smart hub looks better and is a lot easier to use than Windows Media Center or XBMC. Can XBMC still not run Netflix? Don't even get me started on a Linux setup. The best argument for having an HTPC is for gaming, and gaming and Linux don't go well together.