I haven't used the thirsties but compared to flip nighttime inserts, I think they're less absorbent but more versatile. They can be wrapped around baby in the early stages then trifolded and bifolded later, plus you can use the bamboo or stay dry side to suit your needs. They're more pliable too so seen trimmer when on baby.
Just wanted to clarify - they are not as bulky as Flips when trifolded. I am looking for a daytime solution for a 6 month old heavy wetter. Flips are just way too bulky. How would they compare in thickness to the Flip stay dry? Just trying to picture how they would fit. Thanks!