Definitely the Pfaff over the Singer. The Singer 237 has a direct drive, but the motor on these older Singers are "tired." The Pfaff has a chain drive, which gives it more power to get through heavy fabrics.
You may want to consider a Baracuda sewing machine. Here's a link:
https://www.google.com/search?biw=19...k1.rRYF_sXdOCM
Or a used industrial sewing machine like Juki. Check the classified ads on Craigslist or your local newspaper ads online.