
Of the two, fans post want Viggo Mortensen as his Lord of the Rings experience and of any Hollywood actor probably most matches the imagined "look" of Roland which is basically the classic desert worn western movie gun for hire. However, there is a reason he is running in second as getting that kind of commitment from the actor will be incredibly difficult. He also has a rumored history of being difficult on set. This doesn't even include the high salary he can and will demand.
This assumes the report is true. Considering the series is probably at least a year away from beginning any meaningful pre-production, I would be surprised if casting has begun as the first step would be to have at least a working script and detailed outline for the series so actors would know what they are committing themselves too.