Speaking as a previous owner of a NB Miata and a current owner of a 2006 Elise, do NOT test drive an Elise.
Once you test drive an Elise, there is no going back
They are very different cars. I would still buy a Miata(or MX-5) any day but not over an Elise. In fact, my wife is thinking of getting one(2008 HT) later this year.
But I would not get an Elise as my only DD for reasons already stated on many threads here.
If you get the MX-5, you'll always flirt with the Elise. Not true the other way around.