FW = stands for Firewire a interface standard for serial bus. Im not sure if the Montage USB is 3.0 or 2.0.

Depending on what standard Firewire 800/400 vs USB 3.0/2.0. firewire 800 would be faster than USB 2.0.

Im not sure about USB 3.0 vs Firewire 800 you can research or google it.

Montage has no FireWire upgrade, not sure how that would apply to any form of Montage connectivity. The host port on Montage is USB 2.0.

What is the difference between using Yamaha Steinberg FW ASIO and Yamaha Steinberg USB ASIO driver for Montage?
Yamaha Steinberg FW ASIO seems to be the default value when opening Cubase 9.
Which should I choose and why?

The Montage uses the Yamaha Steinberg USB ASIO driver. The only current product still using the FW driver is the Motif-Rack XS. It was used by the Motif XS/XF (both discontinued).

The default for Cubase is whatever ASIO driver you selected last. Make your initial selection from installed drivers on your computer. Find the selection by going to DEVICES > DEVICE SETUP > VST AUDIO SYSTEM > Select as ASIO DRIVER "Yamaha Steinberg USB ASIO" (if on a PC) or Select "Montage" (if on a Mac).

The Yamaha Steinberg USB driver handles both MIDI and Audio for the current crop of Yamaha Synthesizers: MX (2-in/2-out), MOXF (4-in/2-out), Montage (32-in/6-out). (Several of Yamaha Digital consoles and the Steinberg interfaces also use this driver).

Just FYI: The S90XS does not do digital audio (so it uses the Yamaha USB-MIDI driver - MIDI only).

For Montage your choice is the Yamaha Steinberg USB driver which gives you 32 output buses from the Montage to your computer, and 3 stereo returns to the Montage from the computer. Plus two active Ports of MIDI.

http://download.yamaha.com

Select your country
Type in your MODEL NAME: Montage
When the SEARCH RESULTS appear verify the Montage 6, 7 or 8
All the current downloadable items for the Montage will appear. You can further refine your search by your computer OS.

Find the Yamaha Steinberg USB driver for your particular computer.

Speed was and has never really been the problem for USB (versus FW). USB has been fast enough for years... Both have had plenty enough speed for music duties. It is only fairly recently that USB has had the bandwidth to move multiple channels of MIDI and Audio back and forth simultaneously (sufficient for musical needs). Now that that barrier has been overcome, the much more standard USB is being used for Montage and the current crop of Yamaha synthesizers with built-in AUDIO and MIDI interfaces.

Thank you guys for the information. This helps me to understand.
Brau10