The X3M is a really amazing car. I went from the RS5 SB to it. The RS5 looking back was probably a better all around car though. The X3M is more fun to drive than the rs5 or A6 IMO but it comes at a really steep cost. The drive quality is really bad. I've had a m4, m5, rs5, and TT RS, all sports cars, and all had far drive quality. A lot of the X3M owners are really touchy about the subject, and will say things like "You don't know how a M is supposed to drive", an absolute garbage response. The problem is the car is too tall for the suspension set up they went with, and it manifests by how the car bounces up and down on roads that aren't smooth. When I first got it, the first day I had massive buyers remorse as I was being bounced about in the drivers seat. I never had a drink in the cup holders, cause it would splash all over the place. That said, you could take corners at high speeds, and I'm sure it defied logic on the track. But too me it's meant to be a fun sports vehicle that doubles as a family car, and it didn't deliver.
I replaced the springs with MSS springs, and that made a world of difference. I've never lowered a car to find it more comfortable, it changed the spring rate so it wasn't bouncing so much. I was fairly happy at that point. About a year later I was able to get my hands on a used TT RS. Driving it felt wayy more comfortable than the x3m. So i found myself in the place where I wanted to drive the TT when I was alone, and when I was with the family, I was driving an uncomfortable SUV that was bad on gas. When I got the X3M I got a really stellar deal, so when I started to think about getting rid of it, I was really close to a clean swap with the A6.
When I drove the A6 it so so much more comfortable than the X3. It's drastically under powered, and steering isn't anywhere near as sharp. The A6 feels like a big car, the x3, really felt like a sedan. The A6 is a lot more spacious as well. I'm 6'4 and nothing fit behind me in the x3, second row space is better in the A6.
The A6 is quick, but just no where near the s58 engine. The s58 is really an amazing engine, the new M3 is going to be amazing. But my plan is as parts come out, I'll tune the A6, and it'll be fast enough to be enjoyable as a daily, and the TT RS will be what I drive when I really want to play around some. To be fair though I tuned the TT and it still doesn't feel as playful as the x3m, the x3 is more rear biased and torquey, it makes for a fun drive. But that really does fade quickly.
These are my thoughts on it, I test drove the x3m and honestly I was blinded by how good the engine was, and ignored all the downsides. I would really try to drive it several times to make sure it's what you want. If the ride doesn't bother you, then I'd really recommend it. But I love wagons, and haven't regretted the A6 for a second. Every time I look at it, I'm happy. With a few bolt ons, it's going to be my perfect family car, The rs6 is almost certainly better, but I would have been to anal about it at the price point, I don't want to be to bothered by mud in the back seat. Also the adjustable height is a game changer for me. I drive in the lowest setting, but during snow storms, I got to max height, and the clearance is pretty decent. Final disclaimer, there is no theatrics to the A6. It's super quiet, and capable. I need some noise, so hopefully a decent exhaust comes out. So also depending on your taste, you might be waiting for some time for parts. Right now parts just don't exist, and don't seem to be a priority. 034, mentioned they have a lot planned. Hopefully.
Hope that helps.
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