There are a few threads on this topic already.
The conclusion is always the same. Everyone's butt is different, so it will only matter what you think. Just like everyone's ears are different so opinions on whether the stereo upgrade is worth it will range from "you're an idiot if you do upgrade" to "you're an idiot if you don't upgrade".
The only thing which will matter is that you go test drive a car with 18" wheels and ACC, and then another with 18" wheels and without ACC. Preferably at the same dealer so you can drive along the same roads.
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