We stock both, Goodridge was one of the original developers of the fitting design that met DOT standards for the whip test to begin the brake hose market 25+ years ago, everyone else followed suit. I have had a personal relationship with Goodridge that dates back to 1993, when they just had their California location, They have since added their Nascar Development center located in NC late 90's/Early 2000's. They relocated the G-STOP hose fabrication to their California location from NC around 2018-2019 for better product focus and quality control. NC is mainly fluid handling production.
J&M's are an excellent value and made to very high standards as well.
We recommend both, with full confidence and have worked with both companies directly to develop all the recent Mustang lines. On S550's the largest issue we have found has been Ford production quality, over torque of the brake line fittings which stretches the threads on the hard line so it appears the hose is defective. We buy production hoses from Ford and all fittings supplied match OEM hose assemblies. We have had Customers experience hoses only fitting on one location, even though all the fittings are identical. Fortunately this is very rare. Most times using a 10mm x 1.5 nut to "re-thread' the hard line fitting that has been stretched solves 90% of the fitment issues. That does not help the Customers who have removed factory hoses to find the hard line nut completely stripped of threads. The option there is re-terminate the flare with a new nut or opt for a replacement hard line (which are available)
From our experience its luck of the draw, because there is not pattern to fitment issues, we have been selling the S550 lines since 2016 and out of several hundred of just 2-3 part numbers we have had received maybe 6 fitment issues. We are not including an old assembly issue several years ago where the lines were recalled and replaced free of charge. I personal inspect every batch of lines that Goodridge builds for us, we have not had line quality issues in several years. All of our Goodridge lines are made for us directly per order, so we have assembly production dates of every line made which is traceable back to the California Goodridge HQ.
2015+ J&M Mustang GT Non Mag Ride
2018+ J&M Mustang Mag Ride
2015+ Goodridge Mustang
We also have specific lines for GT350/500, GT350 can use the S550 lines without issue, they are just slightly shorter on the front.
The front lines are longer on the GT350 due to the need to push the caliper up to service the brakes, rears are the same part.
While J&M used to be priced very similar, the recent business environment has caused Goodridge to have a substantial price increase going into 2022.
Time will tell to see if J&M can absorb the raw material increases, but they are a much smaller organization with a very lean supply chain vs Goodridge with global engineering and manufacturing presence that supplies OEM's F1, NASCAR etc.