I've heard pilots talking about how the new 737s produce much more wake turbulence then the model historically. They described it as similar to the 757, based on anecdotal experience. Have there been any studies of this, or is there a quantifiable difference between 737 models?
According to the Wake Turbulence Re-categorisation, the older 737's are Lower Medium, while the 737NG is Upper Medium.
The 757's are Lower Heavy. See: What is the cause of unusually strong wake turbulence from the 757?