Many greats points were already mentioned. I went through this exact scenario a few months back. My recommendation: test drive the cars back to back. I had been set on a Panamera for almost three years but was considering the M6 or 650i, too, being a Bimmer guy. I test drove the 650i immediately followed by the M6. Loved both...and afterward I felt like I had just made my decision that much tougher. I drove two miles away to the Porsche dealership (right away), and I test drove the GTS (for the 10th time). I knew right away that the Panamera GTS was for me...but it really took me driving them immediately one after the other to truly know.

One more point on the 6 series--I feel they have the most well designed front and back ends of the Bimmers--gorgeous, but from the side it reminds me of a stock Toyota or Nissan fill-in-the-blank. It just didn't move my meter like the GTS.

Best of luck.

My new GTS just arrived in port. I should pick her up any day now. Exciting times.

 

I am a HUGE fan of both cars. Having once owned a V10 M6 convertible I can affirm that it is a beast, albeit a heavy one...and that's the rub. Because of that weight, I went through three sets of tires in 18 months of ownership. No complaints regarding the M6 though other than you'll never get a chance to fully wring it out on public roads. Having used my P4S as a daily driver for the past three years, and as a former M6 owner, I would choose the PGTS all day long. Uber quick, powerful, nimble, and seats four passengers easily while beings sports car have great appeal for me.

 

I forgot a major point in favor of the PGTS. The standard Alcantera 18 way seats are the best available in any car. No, they do not have the varied and powerful massage functions of the A8 and S Class, but they they are incredibly supportive and comfortable on long drives or in tight corners. As much lateral support that you could want short of racing seats.

 

I am a current owner of a 2013 Panamera GTS and a BMW E60 M5....not an M6, but similar in some respects.

The Panamera GTS is a great car, and I use it as my daily driver. It's very smooth and refined, yet still has the ability to drive very sporty and aggressively in Sport Plus mode.

The M6 will likely out perform the Panamera GTS on the track. It has more power and may be a bit more nimble in the turns.

You should look at the BMW M/// Gran Coupe....it's a great looking car and is more along the lines of a Panamera GTS. I've driven one and I think it's a closer comparison to the Panamera GTS.