As Spash said, the main advantage over the MS is the lower CoG, meaning that the car can be run softer, and hence generate a higher level of grip. This is apparently more noticeable on high grip tracks. At the US reedy race, the MS and MSX were run back to back, and the MSX was simply faster, because it could generate more grip. On lower bite tracks the difference is less noticeable, but still there. For example, Adam Rogers was running his car (an MS) with yellows all round, whilst David was using the HPI blues (which are apparently softer). Nothing too really call between them in terms of pace (Adam TQ, David 2nd... David then won both finals), but Davids looked more stable, and less on edge than Adams.
Of course the easier maintence is also a big bonus...
