I chose to go fitted with covers over AIO/AI2 due to cost. They are economical to buy and you don't wash the cover every time so the PUL will last for ages (whereas an AIO you are washing the PUL every use). Plus most fitted nappies are OFSM.

I am a bit sceptical of only changing the "inner" bit of an AI2 - as I'm sure the wee would get on the outer as well, unless the "inner" was large, practically a fitted nappy itself.

I have a lot of BBH fitted nappies - one bought new and the rest bought second hand. They are great as they fit a wide range of sizes and are very very absorbant. I doubt you'd find an AIO/AI2 that is as absorbant as a BBH fitted. DS wears then overnight (sometimes with a booster if I can lay my hand on one) and they last him 12 hours. Drying time is 1 day in partial sun in QLD weather. Not as trim as an AIO/Ai2 but the pay off is absorbancy.

Happy to answer any questions you have.