I think your friend would be happy with either. The biggest difference was between S1 and S2, with the S2 being far punchier, making the S1 sound slow and muddy. Moving from S2 to S3, the difference was smaller, but they were far closer in sound than S1/S2. I think it would depend on the price difference between the S2 and S3.
Make sure he has a good amp though - the 600 series dips to 4ohms, and needs more control in the bass area than most other competition at it's original price point. The Rotel amps always went very well with them.