The item descriptions don't give away much, and the only difference, on paper I can see is that you can actually Detonator jump (alt fire with Detonator).
What are the main differences between the two weapons? Is there something I'm missing here?
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From the Team Fortress Wiki:
The weapon's main trait, however, is its eponymous detonation ability. While a flare is in flight, pressing alt-fire will cause it to explode, creating a small blast radius which damages and ignites nearby enemy players, additionally destroying sticky bombs. Flares will also explode upon contact with the world, but this explosion will not damage or knock back enemies.
This explosion provides a couple of neat things: