If you have an employer in mind, I would suggest asking what they would prefer, though I would say most I have come across are not specific.
Personally I prefer Madd Dogg Athletics, Inc. (MDA) (co-founded by Johnny G). They are the pioneers. They know their products and from what I can see, a well-rounded corporation. They own spinning.com as well as products like the BodyBlade. They initially worked with Schwinn for their own line of bikes until Schwinn got sold to Bowflex after bankruptcy.
I would say it depends on where you teach and what kind of bikes they have. I have my Schwinn Certification but also trained in Spinning, and Keiser. I have taken classes involving Real Ryder at IDEA conferences…but very few gyms have those bikes where I live…so that doesn’t do me any real good. Certifications are just basics for your teaching skills…a Spin Teacher will tweak their own classes to the tastes and abilities of your class and your gyms bikes. I love teaching with monitors to gauge my students…but where I live very few…actually none….have monitors….so my training in that aspect is not well utilized. If your particular gym has monitors….Schwinn teaches great classes involving monitors. I particularly liked the Lamond cycles…but its been many years since I have even seen them. The instructors for Schwinn are amazing…and at the IDEA Conferences…are abundant. Julz, Jay, Shannon, Kelli, Rob, and on and on….all in all…its about your own style once you have any certification….as far as a bike goes…you could incorporate the training of Spinning or Schwinn Indoor cycling to adapt to whatever bike you teach on….bottom line….when you teach on a bike…you either increase or decrease your resistance…and either increase or decrease your speed/pace/cadence and in or out of the saddle….its not rocket science…mostly class depends on the instructor, their energy and knowledge…and most importantly…the music they choose…..good luck!!!