I've transferred at both in the last 13 months, twice each. I've never noticed a significant difference, though ICN seems bigger, more worldly with more shops if that's your thing. I like NRT because the walking distances are shorter, though less eating choices etc than ICN.
I've honestly never considered English speaking ability at any airport I've traveled. If nothing else I follow the crowd. Also, just like at ATC, English does seem to be the common denominator worldwide.