When designing a software system, architects make a series of design decisions that directly impact the system’s quality. The number of available design alternatives grows rapidly with system size, creating an enormous space of intertwined design concerns that renders manual exploration impractical. We present eQual, a model-driven technique for simulation-based assessment of architectural designs. While it is impossible to guarantee optimal decisions so early in the design process, eQual improves decision quality. eQual is effective in practice because it (1) limits the amount of information the architects have to provide and (2) adapts optimization algorithms to effectively explore massive spaces of design alternatives. A user study shows that, compared to the prior state of the art, engineers using eQual produce statistically significantly higher-quality designs with a large effect size, are statistically significantly more confident in their designs, and find eQual easier to use.