The offer was £7,800 to change! Now that was on a Z4 I paid £27,000 for 6 months ago (and now that M is cheaper than that was when I bought it)! I also own my Z4 (no finance) but I would need finance to buy that M.
Don't get me wrong, if I didn't already own the 3.0si and I hadn't just bought the car, I would have snapped his hand off!
It would cost me around £110 extra on insurance as my 3.0si is only £210 fully comp! I get an average of 27.6 MPG out of my 3.0si as I don't "hang around" when driving the Z4 (the shortest route a week is on A and B roads, hardly any cities of around 45 mile round trip) so If I had an M it would use more fuel! It would also cost me nearly £200 more a year in road tax! I asked the dealer about servicing at which he said it's around 10 to 12 k intervals and the first (oil) service would be the same as the 3.0si. It's after that that the M starts costing more on major servicing.
Power wise, actually, up to around 5,000 rpm I think it felt similar to my 3.0si and a few markers I have on dual tracks (coming out of a roundabout up a long straight slight climb and under a bridge being one) the M was doing around the same speed as my car. At around 5-6,000 rpm the M with the sport button in did def kick harder than my car!
Put it this way, I didn't get out of the M and think "Wow". If I did, and I had hoped it would, it would be sat on my drive, but it didn't feel £7,800 + the depreciation in 6 months better than the Z4 I have now!
My conclusion on the 2 cars is this. The 3.0si is very "road" focused in it's design. The electric powered steering, the suspension set-up and the way the power is and how it comes in is more than enough to keep the grin on the driver on the road, and not a lot will pull away from it on the road.
The M is def track focused in it's design. You need to nail the throttle to see the difference, and although that difference is there, it can't manifest itself on the road properly because of all the road stuff you have to put up with day to day. On a track it would make a difference!
The M is an awesome car, but already owning a Z4 it just didn't make sense to swap. I went out in my 3.0si for a burn this afternoon and I realise I already own an awesome car! And it's all mine with no finance!
Back on the topic, the M has lost more money at the moment than the 3.0si from new. Now being an M car it should start to level out soon and the 3.0si may fall behind more, but the game has changed these days with performance cars slowly being looked on like gas guzzling 4x4's have been. The salesman told me that one of the main things people ask now is what C02 tax bracket the car falls into when they phone up for a new or used car where as that didn't even enter into the conversation before! Strange times! :|