lareger alloys obviously mean larger tyres and thus lesser mileage. However, the mileage doesnt drop so sginificantly that you totally refrain from buying rims.
you may like to compare for yourself at the following address:
http://www.miata.net/garage/tirecalc.html
as for the beauty part, well, it all depends on ones taste. there are barely a few which i like rest are all....but if someone has them, it means they like them so that's totally something personal.