The unicorn has guaranteed drops of tokens that can be used in future hunts and also spellbound dew. The mithril dragon , actually it's called a Wyvern , does not have guaranteed drops of mithril. At least I don't think so. I just got one and I got jewels from mine. I got my 4 unicorns before I got the Wyvern.
You might want to check out the Hub for other things that could drop mithril.
The Wyvern is very neat looking though.
CORRECTION: The token drops are not guaranteed, but the dew is