Is there a wake turbulence difference between B737s?
According to the Wake Turbulence Re-categorisation, the older 737's are Lower Medium, while the 737NG is Upper Medium.

High lift (weight) and low approach speed increase the wake turbulence. The newer 737's are heavier, while their approach speeds are not much faster than the older models.

The 757's are Lower Heavy. See: What is the cause of unusually strong wake turbulence from the 757?