The sensitivity of the principal stress difference (PSD) profile to molecular architecture is demonstrated for flow in a cross-slot geometry. For materials with low levels of branching, the pattern along the outlet centre line exhibited "single cusps", while an increase in molecular branching was found to lead to "W cusps". The formation of these W cusps was found to be independent of extensional rate for the conditions probed, and they were formed initially at the stagnation point and travelled along the outlet centre line with time. Comparison with simulations performed using a multi-mode "pom-pom" model failed to predict W cusps, although the general level of PSD was accurately captured.