To be honest, neither 5-APB or 6-APB are particularly clean. They're very very messy. And there really isn't enough difference between the two substances to really definitively state which is 'better'. It's like comparing different types of apples. People will have their preferences, but it's not set in stone as to which apple is the greatest of them all. Neither of them is particularly mellow, they're both powerful stimulants.
MPA is very fiendish, very dirty, and does a lot of nasty things to your body if you try and binge on it. I've had severe chest pains, constant palpitations and protruding veins. I do owe some of the greatest orgasms of my life to MPA, but a side effect of it releasing so much dopamine is that semen is thickened, resulting in excruciating pain in your testicles. The testicles themselves swell, whilst everything else down there shrinks. It's a really toxic chemical. I wouldn't recommend you try it.
MDAI feels weird, I absolutely hate the stuff.
Camfetamine is a good clean stimulant though. I would recommend that.