I've crafted both, and my impression is that Alex is more often indispensable in the decks where she's run, but Sylvanas may fit in a slightly greater number of decks (although I use Alex in more decks currently). So I guess if you play or want to play decks that Alex would fit in, I'd go with her first. I don't think you'll regret your decision either way, as both are great cards.
ETA: currently I use Alex in 2 warrior decks (a fairly standard control warrior as well as a more hybrid warrior), a mage deck (not freeze), and a shadow priest deck. and I don't play either, but Handlock and Freeze mage also almost always run Alex. there are probably others, but there's a pretty good lists of decks that commonly use her, if you needed it.
Recently got 1600 dust. Which one should I craft first: Alex or Sylvie?
I mostly play control decks, so both are a good fit. But which one should go first?
EDIT: I have Rag and Cairne. The first 2 must-haves for control decks.
Sylvanas, outside of Freeze Mage, Alex is a bit too slow for the current meta right now.
I've crafted both, and my impression is that Alex is more often indispensable in the decks where she's run, but Sylvanas may fit in a slightly greater number of decks (although I use Alex in more decks currently). So I guess if you play or want to play decks that Alex would fit in, I'd go with her first. I don't think you'll regret your decision either way, as both are great cards.
ETA: currently I use Alex in 2 warrior decks (a fairly standard control warrior as well as a more hybrid warrior), a mage deck (not freeze), and a shadow priest deck. and I don't play either, but Handlock and Freeze mage also almost always run Alex. there are probably others, but there's a pretty good lists of decks that commonly use her, if you needed it.
"Sometimes you eat the bear, and sometimes the bear turns into a cat and eats you." - Anonymous Druid
Moved to Card discussion.
Probably Sylvannas now, Alex is quite slow for the current meta.