...performance wise, with similar suspension and powertrain set-ups, they're essentially the same car - the elise is a little faster in a straight line, the exige is a little faster through the turns, but those differences are negligible compared to driver ability...
...the exige carries more of a hard racecar poise, the elise more of a touring roadster poise - i suggest basing your decision primarily on which character best suits your desired role for the car...