I have been to the high ground via both of these routes. I just have an off-road bike without any extras and was fine on the path up from the start that I made close to the Memorial on the Whitewell road. Of the two ways the there is a path up to around 900 metres on the Dhondail route and probably around 600 metres on the Coire Ruadh/Lochain route.
Round trip? Up one and down the other.
As for roughness and the stream when I went not much of a problem.