Jodie - if you do fitted & covers - try to get fitted that are OSFA - like baby beehinds - they fit both kids really well here. Plus you can't go past motherease rikki covers - they are fantastic - expensive - but well worth it. Bummis are great covers too.

Shell - I have noticed some brands better on DD than DS for example:-

Monkey Doodlez AIO - great on her, suck on him
Bubblebubs AIO - great on her, suck on him
Baby Beehinds AIO (magicalls) - great on him, suck on her

Good on both kids - BumGenius (pockets - one size fits all), baby beehinds fitteds, and itti bittis.

I also try to get nappies (if they are sized nappies) to get ones that have a huge weight range - for example, widdlewuns - mediums are from 7kg to 15kg or something like that - so thats like a year that they could be in them.... what a great nappy!!!! DS can wear both the medium and the large (large is 10kg plus) so while I'm waiting for her to get big enough, he can wear them

However, having said all that - its all trial and error to what suits your kid - cos all babies are built differently.

Good luck!!!