wow.. little confusing.. this isn't a matter of 205 and 215 the 2 tires will fit the wheel. its the wheel pattern that is the problem.. 15X6 means 15 inch wheel with 6 lug holes.. you can't mount that wheel onto a 5 lug...
you have to be sure of what you have exactly.. it could be a 5X 5.5.. trailer sizes get confusing.. refer here to answer your questions..
Determining the Bolt Pattern of Your Wheel | etrailer.com