  The analysis of the broad iron line profile in the X-ray spectra of active
galactic nuclei and black hole X-ray binaries allows us to constrain the spin
parameter of the black hole. We compare the constraints on the spin value for
two X-ray sources, MCG-6-30-15 and GX 339-4, with a broad iron line using
present relativistic line models in XSPEC - LAOR and KYRLINE. The LAOR model
has the spin value set to the extremal value a=0.9982, while the KYRLINE model
enables direct fitting of the spin parameter. The spin value is constrained
mainly by the lower boundary of the broad line, which depends on the inner
boundary of the disc emission where the gravitational redshift is maximal. The
position of the inner disc boundary is usually identified with the marginally
stable orbit which is related to the spin value. In this way the LAOR model can
be used to estimate the spin value. We investigate the consistency of the LAOR
and KYRLINE models. We find that the spin values evaluated by both models agree
within the general uncertainties when applied on the current data. However, the
results are apparently distinguishable for higher quality data, such as those
simulated for the International X-ray Observatory (IXO) mission. We find that
the LAOR model tends to overestimate the spin value and furthermore, it has
insufficient resolution which affects the correct determination of the
high-energy edge of the broad line.
