Just depends what you need/want. We have 2 little ones and a small dog and everyone/everything fits fine in my F80 and our Macan. Is this going to be a second car in your garage or just the only one? If the only one, I'd say consider the F97 since you get a bit more utility out of it, but honestly unless you need vertical load space or extra ground clearance, the M550 could suffice. Also has a nicer cabin, more options, and a bit more overall power, but it's not a motorsport-tuned chassis or engine, and so not the same kind of power delivery as the S58. I would test drive both and see which one feels better. The F97 is a hoot to drive, and it may bring back a lot of the special "M feeling" you're craving. After driving a couple of BM3 tuned versions, I was impressed. A lot of folks here came directly from the F8x platform to this, and are very happy. The M550 is fine, and powerful, but not the same bonkers driving experience. Less fun imho.
Just be wary of demo cars. They take good care of them, but they could always have niggling issues that require dealer visits. Would be covered under warranty, but can be annoying. I would haggle that price down if I could. And what's the $5k? MSDs? Cap cost reduction? See if they'll give you this deal AND eat your last two GC payments, for example.
Good luck!
