The difference is that if you evolve at level 180 you will receive no blessed tomes bonus. These tomes can be used to either level the evolved hero or any other hero you desire, or they can be sold for Honor Badges. With one of the most recent updates you can now use these tomes also to level up the equipment on your heroes for augmentation. Waiting until level 200 will award you approximately 263 blessed tomes.
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To Piggyback off what First Lady said, use that to your advantage. You get 263 purple books aka Blessed Tome II, which are 500k EXP or 12K Honor badges or 100K Aetherocks.
So when you evolve one hero, you can now obtain their weapon. To level their weapon to level 20 (max level before you can augment), it will cost approximately 4 million Aetherocks or 40 purple books.
it will cost you about 24 million EXP to to get your hero back to level 160. 48 purple books (but you won't use all purple if you have the smaller Tome books - green blue & pink).
It will also cost you about 200K in honor badges to get back to 8 star level 160. That will cost another 17 purple books.
So truthfully, after you Evolve a hero that was at 200. You'll only spend about 105 purple books out of 263. So, if you had another hero that was level 180 10 star and ready to be evolved, you could level them right up after evolving.