This paper aims to find the difference between two types of probabilistic slope stability analysis under a rainfall event. The first admits a uniform soil and takes into account uncertainty of the soil properties. The second includes spatial variability of the soil properties. Both analyses are performed using a Monte Carlo framework with the help of a meta-model, in particular a Polynomial Chaos Expansion (PCE). The PCE is built based on 2D finite element realisations and allows a quick evaluation of new ones. This provides an efficient reliability analysis when performing the Monte Carlo Simulation method. A comparison between the probabilities of failure between the two approaches is shown. Furthermore, the effect of the correlation lengths of the random field on the probabilities of failure is evaluated.