you could also ask what edge the Mi-28 has over the Apache
but the Apache is battle proven. Mi-28 is cheaper and more durable. it has a more powerful cannon I think. It carries more missiles and rockets.
if the Mi-28 had been used as extensively as the Mi-24 hind has it might of been more popular.
The Havoc has a somewhat greater load as well. Compared to the Apache , which can carry either all hellfires or a combo of 8 hellfires 2 Rocket pods.. the Havoc can carry 16 Atakas and 2 rocket pods along with 4 A2A missiles.
So it really is a big heavy beast...with the added somewhat advantage of its pilots being able to bail out(if they are actually able to make that altitude to survive) if they need to.