  The Higgs sectors of supersymmetric extensions of the Standard Model have two
doublets in the minimal version (MSSM), and two doublets plus a singlet in two
others: with (UMSSM) and without (NMSSM) an extra U(1)'. A very concise
comparison of these three models is possible if we assume that the singlet has
a somewhat larger breaking scale compared to the electroweak scale. In that
case, the UMSSM and the NMSSM become effectively two-Higgs-doublet models
(THDM), like the MSSM. As expected, the mass of the lightest CP-even neutral
Higgs boson has an upper bound in each case. We find that in the NMSSM, this
bound exceeds not very much that of the MSSM, unless tan(beta) is near one.
However, the upper bound in the UMSSM may be substantially enhanced.
