  The results of simulations of the extragalactic propagation of ultra-high
energy cosmic rays (UHECRs) have intrinsic uncertainties due to poorly known
physical quantities and approximations used in the codes. We quantify the
uncertainties in the simulated UHECR spectrum and composition due to different
models for the extragalactic background light (EBL), different
photodisintegration setups, approximations concerning photopion production and
the use of different simulation codes. We discuss the results for several
representative source scenarios with proton, nitrogen or iron at injection. For
this purpose we used SimProp and CRPropa, two publicly available codes for
Monte Carlo simulations of UHECR propagation. CRPropa is a detailed and
extensive simulation code, while SimProp aims to achieve acceptable results
using a simpler code. We show that especially the choices for the EBL model and
the photodisintegration setup can have a considerable impact on the simulated
UHECR spectrum and composition.
