I wanted my new amp to last another decade at least. Part of what sold me on the Katana-100 was how easy it was to actually modify the amp if you needed to do any sort of repairs - for example, replacing a water damaged cone wouldn't that have been handy Line 6? On top of stellar repairability (and some really freaking cool grill cloth customizations), the modeling cabinet that can be controlled with computers OR phones using an OTG-USB cable, with an Effects Loop, and 3 ranges of wattage so you can play at home and cut through on stage. Now, I'm not playing on stage these days, so the 100W and the HeadCab Switch are a bit overkill for my needs, but the Katana-100W does have some extra inputs for a footpedal (the GA-FC) and an expression pedal that made it the clear choice. 

It would be a little silly to get an amp with an expression pedal jack and decide to not use it... I went with the EV-5 because it has a good sturdy build quality without breaking the bank. You can use this on the Katana-100, or on the GA-FC, which means technically you can have two expression pedals in tandem and set them up for independent control within Boss Tone Studio. 

I took a little time to get the GA-FC because I wasn't sure just how much I needed it. It became clear pretty quickly that if you plan on interacting with the Katana-100 in any way while you are playing, you'll almost certainly need the GA-FC (or the Airstep Katana). The GA-FC turns controlling the Katana-100W into operating preset pedal configurations, allowing you to switch between presets and turn pedal effects on and off. It's also got a few extra inputs for bringing expression control closer to you on stage. 

Finally, the Airstep Katana Footswitch meets right in the middle on the GA-FC -- they actually complement each other very well. The Airstep Kat was designed with GA-FC owners in mind, but also has a mode to utilize some of the control functions that was designed for that device. The Airstep Katana itself also brings a super incredible wireless configuration, as it allows you to connect your phones Boss Tone Studio directly to the Katana-100W via BlueTooth. Granted, a USB+OTG cable are far cheaper, but being able to send patches wirelessly really increases the accessibility of making adjustments on the go. I don't know if I would suggest one or the other more, I would say if you are interested in utilizing the modeling cabinets patching capabilities a lot more, the Airstep Kat would be the better option. If you are more interested in the hardware side and not interested in the app, mostly using expression pedals, then the GA-FC may be the better choice.

One thing to keep in mind is that the Airstep Kat is just a little bit cheaper for just a little bit less hardware (input) functionality, functionally they can be identical. And as mentioned, they do pair really well together, so you can always get both and return the one you don't like as much.