They're cosmetically identical. I understand that there are some differences in construction and welds and such. The 115 is actually a little tougher because a good number of 115s were destined to livery duty.

Other than that the 114s have 6 cylinder gassers and the 115s have 4 cylinder gassers or Diesels.

I'm sure folks more familiar with the series can provide more specifics such as what options were available and how many rows of bumpers to expect in each lines.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD