Hmmm - I can understand the temptation to have an ///M

.
Much as though I really like mine, if I had a 3.0 I don't think I'd change it for an///M that was older and had done a lot more miles.
Even if the ///M had a lot more toys in it than your 3.0 (it seems to have cruise but no nav, other than that it's hard to say) I still think I'd be put off by comparative age and mileage.
At then end of the day it comes down to whether you think the ///M will be sufficiently better to drive to justify the cost to change cars and the extra running costs.
Only way to find out is to have a go in it. I made that mistake and was hooked before I even sat in the car - the sound of the engine starting from cold was enough to do it for me :evil: . Plus, it is quite nice to drive as well

. You have been warned

.
For what it's worth, I still have a silly grin on my face from buying my ///M around a month ago. It is still firmly in place despite reading of average mpg for the 3.0 of 35 mpg or more (I'm getting about 27 from the ///M, but it's only done 1,400 miles so I'm still treating it fairly gently). I hope it remains in place when I next have to stump up an extra £150 tax, £200 insurance, god knows how much on tyres and a shed-load on servicing compared with the 3.0 SE I thought about buying.
But then I wouldn't have been thinking about buying any kind of Z4 if I'd really wanted budget motoring, so what the hell :evil: