The change to the 73+ disks pushes the wheels out about 3/8" per side.
The 15x7 would be the better choice of the two, the 15x6 has a negative offset, so the backspacing is limited. Even the 15x7 with 4" of backspace (0 offset) will limit your tire choices. I ran a 225/60/15 for awhile on 15x7's with a 4.25" backspace on my '74 Duster with disks, and those would not fit without rubbing on a 15x7 with a 4" backspace. I barely had any extra room, let alone a 1/4".
215/60/15 will be the widest you can go on a 15x7 with a 4" backspace.
