I would keep the 911. The 911 and the i8 are two different cars, with two different purposes (assuming you dont' view driving a utility
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The i8 is a fun car to drive and few will be able to test its limits. Its amazingly quick, efficient and a head turner. If you like doing track events, the i8 is not really a track car. That's the 911's wheelhouse.
For the short run, keep both, then see for yourself.