Two of the guys with whom I go driving on Sundays have V10 Audi R8s. They are great cars, but they are to the Elise, what an Apollo rocket is to an F15 fighter jet. Sure the rocket is more powerful and faster, but the F15 requires more skill to drive, and is more rewarding. When you test drive the R8, you might be lured in by the initial reaction to the comfortable seats, easy steering, luxurious leather-wrapped dash, raw power, and creature comforts. The danger is that after a few weeks, all those advantages will make you feel numb, and you'll wonder if you're missing the whole point of a sports car, which is to be in charge of, feel, and enjoy, the driving experience.
I'm fortunate enough to be able to own both a raw sports car like a supercharged Elise AND a refined piece of powerful engineering like a Porsche 911 4s. So if you can purchase the R8 AND keep the Elise, do it. If you can only have one, and you need it as a daily driver, pick the R8. If you can only have one, and it is to be used primarily for fun, I say keep the Elise.
Hope that helps.