Systems codes assess the viability of fusion reactor designs by using simplified models for the entire reactor system that allow for the exploration of large areas of parameter space. However, every design will have an associated uncertainty that arises from the accuracy of the models used, the assumptions made and the values of input parameters adopted. For individual codes, the uncertainty on their results can be quantified by investigating the dependence on the combination of input parameters. More generally, different codes can be compared against each other to test the underlying models. In this paper we compare the results of two systems codes, SPECTRE and PROCESS, using a conceptual design for the SST-2 fusion reactor for benchmarking. We find that overall both codes produce similar results, however different plasma temperatures and densities are found due to the treatment of radiation in relation to the L-H threshold, and different fusion reaction rates. We then apply a Monte-Carlo based uncertainty quantification tool using PROCESS to find that while the design can produce in excess of 100MW of fusion power, it is unlikely to produce pulses over 400s. This is in agreement with previous work and suggests a larger aspect ratio is required.