I love my 'stuff it' bag. IT is only about $20 and fits lots in it and it is light. It goes accross your body and I have used it with DS in the pikkolo. I am going to buy an ergo backpack though.

I would buy a nappy change wallet, a stuff it bag and a 'normal' backpack.

Taht way you have all situations covered. All you really need from a nappy bag is maybe an insulated pouch for bottles which you can buy separately and maybe a nappy change pad. I like my large 'fridge to go', great for his food at daycare.
I have the GR8x Nappy change wallet and it clips on to either the stroller handles or around the stuff it bag but does make it bulky.

It all depends how much stuff you will need to take with you. Will you be driving to a shopping centre so will be able to have a bigger bag in the car and just the basics or do you plan on leaving from home with bubs in the carrier.