While a democratic Japan does give you your freedom for free and give you that sweet Militarism buff, it has drawbacks. For one thing, democratic Japan behaves like an Asian UK, in that they'll just start obnoxiously guaranteeing things, and trying to protect China from you. Also, you get Mengkukuo as a puppet, which means you must annex it through the autonomy system if you want to claim the mandate of heaven. Democratic Japan also keeps East Hebei and Dalian, so you're gonna have to attack him for those. You can totally just gank him for those provinces while he's still fighting the civil war, but you're still fighting Japan just like you would if you declared independence, only you don't annex Mengkukuo directly, and Puyi doesn't get the Nationalist Symbol trait.
Finally, you don't get the satisfaction of doing it the hard way. The "hard" way isn't even really that hard. Train a dozen 20 width cav divisions and then spew 20 width infantry until Japan has pushed China back to around the Yellow River. Declare independence, rush Korea with your cav, gank his ports and watch his army collapse. It's pretty straightforward once you know the strategy.
Oh, and once you're independent, all those "dick with Japan" focuses that the various China factions have now dick with you. So watch for that.
