Why, funny you should ask! The Fly 3 has been my portable amp of choice for a while now.
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Spent some time playing both side-to-side yesterday. tl;dr, i think the Spark GO sounds
great for a battery powered practice amp. The Fly 3 sounds a bit bigger, like there's more actual air being moved, but it is also definitely boxier. Whereas the Spark Go is more like a finely tuned general purpose speaker, and can thus cover way more ground. One of the presets it ships with is Dual Recto-inspired, and it is remarkably convincing.
The Spark Go is also smaller, and feels very very solid. They packed a lot of things into a tiny volume, including a tweeter and a small full-range transducer. The Fly 3 has a more classic, single-speaker-in-a-cabinet approach, and it's bigger - but lighter without batteries.
Both can get quite loud. I haven't driven the Spark GO full volume, but i got it loud enough to annoy the wife next door, and i couldn't get the speakers to fart at all.
Feature wise, the Spark Go wins hands down. Even if you never care about using the app, the fact that it can recharge over USB-C, doubles as a Bluetooth speaker, and has a (not great, but serviceable) tuner makes it IMHO a much better portable solution than the Fly. The stock presets are quite good, and you can easily just get by with those. The USB audio/headphone out is also on a different league than the Fly; i'm not too fond of Positive Grid tones in general, but you
could record these just fine. Effects are quite good though, and sound great through this amp.
Overall, i think it's a winner - at least for my use cases. And at a hundred bucks, it's a steal overall. I've already listed my Fly 3 for sale.