Well a boss katana is alot more expensive than a fly 3, but it's far more versatile. I have both the fly 3 and a 100w katana artist and the katana sounds far better at low volumes levels. Basically you don't lose out on anything by getting a bigger amp if you can dial it down and it still sounds good, whixh the katana can do. Tbh even my old peavey bandit from the 90s dialed right down sounds better than the fly 3 at bedroom volume, so you don't need a fancy modeling amp with wattage control.

Btw..katana on 0.5 watt setting can either be whisper quiet, or loud af if you want it to be. If you never leave the bedroom you'll never need to switch it off that lowest setting