I feel somewhat qualified to answer just because I own both cars. The Miata has given me 53,000 completely trouble free miles. The Mini has had a number of things replaced and still has a number of pesky, but minor, problems. Both are terifically fun cars to drive but not all that similar. Depending on where you live figure on adding $1500 to the price of the Miata to add a hardtop for winter months. '04 model changes for the Mini will undoubtedly be small and price increases, while potentially minor, seem almost inevitable. Great source for Miata info is
http://www.miata.net
If you have specific questions about strengths/weaknesses of both cars feel free to pm me.
Dave