What swung it? Sportiness?
I do about 20k miles a year and have to pay my own fuel. The Z is a little better on economy and I really like the idrive system for overall connectivity. I also prefer the DCT gearbox over a regular auto.
If I wasn't doing the mileage an SL55 would have been on the cards.
The E89 is more of a mini SL class than its rivals.