  Recently the full O(alpha_S^5 m, alpha_S^5 m log(alpha_S)) correction to the
heavy quarkonium 1S energy level has been computed (except the a_3-term in the
QCD potential). We point out that the full correction (including the
log(alpha_S)-term) is approximated well by the large-beta_0 approximation.
Based on the assumption that this feature holds up to higher orders, we discuss
why the top quark pole mass cannot be determined to better than O(Lambda_{QCD})
accuracy at a future e^+e^- collider, while the MS-bar mass can be determined
to about 40 MeV accuracy (provided the 4-loop MSbar-pole mass relation will be
computed in due time).
