Stansted is pretty good yeah... East Midlands is probably slightly closer, but Stansted has a good train service with direct trains to Peterborough so it probably wins .. but if you plan on driving then either would be good (Luton would be okay if driving too)
Lived in Peterborough for 6 years was a regular commuter home! Stansted is considerably faster drive straight up the M11 to the A14, after that Birmigham M6, A14, dual carriage way the whole way..
East midlands way off, bad roads, longer drive, not to mention the infrequent flights!