I think you'll be happier with the 124. I've had both, with the M103 six and I prefer the 124. In fact, I just got home from a road trip to Colorado. Yesterday I drove 900 miles with only three stops for gas and food.
IMO, the 124 rides better than my 126 did. It is quieter and smoother. It's nearly as big inside and the trunk holds just as much if not more than a 126 and it's much easier to lift stuff in and out.
I was surprised at how much better the 124 felt on the road. I guess it's because the engineering is about 5-6 years newer in the 124. I also got between 27-28 mpg on the highway. The 126 never got more than 22-23.
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