Aachen to both CGN (1-stop) and DUS (nonstop) by train is 91 and 96 minutes (don't be fooled by the current situation, there are engineering works at Cologne main station at the moment). MST seems to be the most logical choice, also given that it takes 2,5h to get to Schiphol. Nevertheless, all scheduled services to "city" destinations from MST have miserably failed over the past decade - Easyjet, Ryanair and Germanwings have all tried since KLM exel disappeared. Even Ryanair's leisure destinations have not been stellar performers.