At the end of the day there are 4 postcard sized bits of rubber keeping you on the road - never skimp on tyres, get those budget ones off as soon as you can, Falkens should do you well 
Brasseye said:My BMW winter e89 wheels recomended are 17" 225 all round ?
Bing said:Worth also saying that whilst the Z is set up for staggered wheels generally, non sport models came with a square set up I believe. Also, it's set up that way for performance because it's a sports car... But you will not be driving it like one in the winter, so there is not need for wider rear tyres. Whilst I defer to the above poster's considerable experience, if you ask a BMW dealer what the recommended winter set up is they will tell you it is 225/45/17 all round. And I was of the understanding that skinnier tyres are better in the snow than wider ones... If I'm wrong then so be it, but running square 17s on my last Z with winters worked just fine for me. Ugly as sin wheels, but I never had any issues.