I traded a Z4 in on my SRT6. The Z4 is a very nice car, however if you read what the experts say, they haven't been overly kind to the Z4. While the Xfire's steering may not be quite as precise as some others (I personally think it is just fine) the Z4's electric PS has been chided regularly for lack of feel. No question the base Z4 coupe will outperform the base and limited Xfire, as you put it, it does cost a lot more as well.

My advice, get one of those fire sale SRT6s. You'll save about $15K and get a much better car. Now of course if you would rather spend an extra $25-$30K for an M Coupe, they might compare pretty close. But then again for the same money you could have both an SRT6 roadster and a SRT6 coupe.