Hello and Welcome.
Based on the comment “as my riding area include flat roads in good/very good condition only” (so no hills at all?) — I would vote for the SX with the 18" wheels – it makes a significant difference over the 16".
I am NOT familiar with the MAS but with 2 speeds if there are hills this could help. As you mentioned, there is more complexity and if it’s keep things simple on flats - SX should do it.
Of note is I live in Singapore - great weather and mostly flat terrain but the occasional hill or overhead bridge. The 3-speed EVO on low gears tackles inclines easily and zooms on 3rd top gear at approx 25 km/hr with easy peddling effort. I admit it will be harder to find a 2nd hand EVO if this comes into the consideration factor.
My rider’s report is here for the Strida EVO. [url]Photos of Stridas in Hong Kong]