A few thoughts after entertaining both briefly:
The 2008 1series was the first year for that I believe and had some reliability issues.
The 1series is more practical for short drives with the rear seat. But that seat is cramped.
Both are very fun and the looks are subjective. I preferred the roadster obviously but the 1 series is nice as well.
Depends on what you want. I'm able to use the roadster as it's my commute car and I rarely get the kids. My wife does most of the hauling in her Forrester.
You won't see many Z4's on the road; 1 series are much more common.
In the end the Z just felt right for what I wanted.
