Eventhough neither one of the 126 nor the 124 that I own are sixes, I want to add my two cents.
The two are totally different. The 126 is big....very big. 17 feet of big. It will handle big....at least compared to the 124. To me the 124 is nice and nimble. The 124 has less body roll.
I would think about what kind of driving you plan on doing. Lots of turning? Are you going to be parking in narrow parking slots? Do you like to have alot of room in the engine compartment to work on it? And, how much support do you want in the seats? The 124 seats hold you in place much better than the 126 seats do.
For cruising down the highway in an unhurried fashion, the 126 is great. If you're in a hurry, and plan to zip around town, and possibly on the highway, then the 124 would be better suited.
Another way to look at it....is their personality. They are distinct.
One is relaxed, unhurried, yet still makes its presence known, and has a unique air about it. The other is more down to earth, a little less pompous, and would rather roll up its sleeves to compare tattoos than ask for some grey poupon.
"...rationality of thought imposes a limit on a person's concept of his relation to the cosmos." -Nash
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09 Jetta TDI