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I think both have their ups and downs.
Mega Evolution is cool because you're essentially just using a powered up regular Pokemon, so you can still get creative with their learnsets. Dynamaxed Pokemon can only use standard Dynamax moves so it feels more disconnected from the regular Pokemon.
That said, Dynamax has a fair bit more advantages of its own - the fact that it only lasts for three turns does give it a more strategic edge over Megas.
Another good thing about Dynamax is that every Pokemon is allowed to use it, rather than just the ones that were chosen to get new designs.
And to get away from competitive for a second, Dynamaxing is much more fairly distributed in the main game than Megas ever were, which makes story battles feel less one-sided.
