Looks like apples to oranges to me. I own both a 95 Springer and an 06 NC Touring. I love 'em both but what comparisons can be drawn directly?
Harley = better mpg with regular unleaded, lower insurance, lower depreciation, HOV and parking advantages, better driver's sight line, quicker acceleration
Miata = better cornering and top speeds, better protection (elements and accidents), better cargo capacity, stereo, climate control, passenger conversation
One common element = chicks dig 'em!