The SOLPS-ITER simulations of European DEMO reactor, with 450 MW of power to be exhausted, show an increased operational space compared to the conventional single-null configuration. Using a reduced model with fluid neutrals and bundled impurities, we assessed the existence and the boundaries of operational space for DEMO with the conventional single-null divertor and DEMO with a super-X divertor which has larger major radius at the outer target and increased connection length, by carrying out fuelling and argon seeding rates scan, as well as input power scan. Compared to the conventional single-null divertor, super-X divertor is found to offer a larger margin of impurity concentration (factor ~2 lower) at the same main plasma density, and consistent with this, it has lower main plasma density at the same impurity concentration. This observed difference is in line with the simple analytic Lengyel model predictions resulting from the increased connection length in the super-X configuration. Demo with a super-X divertor demonstrates remarkable robustness against increases in input power, and in this study was able to safely exhaust the maximum expected steady-state separatrix power of 300 MW, something that was not possible in the single-null configuration. This robustness of super-X divertor lies mostly in its capability to sufficiently dissipate power in its divertor via argon radiation, which is related to two factors: longer (than single-null) parallel connection length from the upstream to the outer target and higher extrinsic impurity concentration at higher input powers. Finally, consistent with neon-seeded simulations of ITER, it is observed across all the simulations that the plasma density drops with increasing amount of argon in the plasma. We found that as argon content increases, the accompanying enhancement of argon radiation reduces the power available for deuterium recycling, hence limits the deuterium recycling particle source, and consequently reduces the plasma density.