Ryley is definitely better.
Sydney just seems overdone and the nn
Sid, as someone else said, isn't befitting for a girl. Don't know what spelling I prefer, though --
Ryley or
Riley -- but definitely not Ryleigh -- that's too much. What about Rylie? Some middle names ...
Ryley CallaRyley MargaretRyley FirenzeRyley MorganRyley PiaRyley AbigailRyley CiaraRyley Maeven
like any of these?