They're both very lovely. It's a tough, tough call, but I think I prefer
Will just a tiny bit, perhaps because I really love the association to the word "will" (even though that's not where the name is derived from). I really like
Rowan, too, though, so either would be more than fine with me.
Will and
Rowan would make a perfect sib set, BTW, and a fantastic combo, too.
Will Rowan as a FN & MN combo would be utterly gorgeous, too.