While I would normally lean towards the concept of separate boxes, with the idea of 'jack of all trades, master of none', my own experience was just the opposite.
We got a Vizio Smart-TV last year, and we have ROKU boxes on two other TVs. Initially, the Vizio had problems with Netflix, but they finally resolved that (firmware can be upgraded over the 'net). Recently, Netflix finally allowed multiple profiles - this was important as DW did not want my documentaries in her queue and I didn't want her 'dramas' in mine. Surprisingly, Vizio updated and works with the separate profiles, ROKU says sometime next year.
If you get the Smart-TV, you can always add a ROKU box to it. They aren't that much, and I don't think the price difference of smart/dumb TVs is much either.
Though, ROKU does mean one more remote. Oh, that reminds me - the 'instant replay' of the last 10 seconds seems to work better on ROKU, and that is its own button.
-ERD50