I really like
Susannah too, I much prefer it with the 'h', it looks more complete.
Susan's ok. It does feel a little dated to me, but my initial association is with 'Little Friend
Susan' in the
Milly Molly Mandy books so it's a little girls' name to me too.
I don't know whether
Susan will come back in in the near future, it's still a 'mum's' name IMO. I can
Susannah happening though, it's quite sweet.
Susie would be the most common nn I think, although Suki might come in too. Personally, I prefer
Sanna.
I prefer
Adelaide to
Adelaida (also,
Adelaida and
Susannah might seem a bit matchy with the same endings), but
Adelaida (
Ada) and
Susannah (Zuzu) would be cute sibset.