If the point is to write legible, explicit code then the extra 4 characters of "this->" over "m_" shouldn't be a deal breaker. It also has the advantage to save space and reduce noise in locations where you really don't need to make it explicit (like declarations in particular where you obviously don't have the "this" but you'd need to add the m_ prefix).

If the point is to write super dense code for some reason then don't use anything at all.