You need to let the wife drive the car before making any decisions.
While there is a big difference in the level of car here, the size difference is tremendous. Shopping and around town stuff, you can't beat the nimbleness of the 190.
Also you said concerning the tight budget, A S class has a lot more things that might require replacement at a substantially higher price than the 190.
When you pull up in a S Class the first thing peole think is you have money to burn. In a 190 they might think you are money smart and frugal with it.
The difference in Fuel usage is 4-6 gallons at least. While on the hiway the differences in gas are lessened, around town is a different story. Not sure where you are from, but here in the midwest we are going through our usual screw by the Oil companies. I've had my Sunoco raised by $ .10 in less than a weeks time. Up 30+ cents over the summer.
71 250 Coupe
92 400E Sedan
90 944/S2 Cabriolet