I think Manchester United, if sold as a project would be a dream job for a manager like Pochettino. However, Pochettino is an extremely loyal person, and also doesn't seem to be someone who would be satisfied leaving the current project early. If he was a manager of neither club, and could pick between joining Spurs (the current one or the one he joined) or United I think he would pick United. And I even think the idea of United would be a more tempting prospect than Real Madrid, since he would not be given time and freedom to treat it as his own project.
However, I do believe that he will stay with us a while longer until he feel the project is over, it may be 2 years, it may be 20 but I don't see him leaving just yet.