I have just swapped my 28i for the 35is and the difference is amazing.
I was very happy with the 28i which gives great performance as well as being cheaper to run however the 35is is on a different level. A friend of mine drives a 360 Ferrari and when he drove mine andput his foot down his first words were "jesus this is quick". If you go down the 28i route then my advice would be to try and get a model that includes the adaptive sports suspension it does make a difference and is worth the extra. The manual version of the 28i is also the one I would recommend but the dual clutch in the 35is is by far the best auto I have ever driven so unless you have that gearbox stick with the manual in all Z4 models.
I was very happy with the 28i which gives great performance as well as being cheaper to run however the 35is is on a different level. A friend of mine drives a 360 Ferrari and when he drove mine andput his foot down his first words were "jesus this is quick". If you go down the 28i route then my advice would be to try and get a model that includes the adaptive sports suspension it does make a difference and is worth the extra. The manual version of the 28i is also the one I would recommend but the dual clutch in the 35is is by far the best auto I have ever driven so unless you have that gearbox stick with the manual in all Z4 models.