The new Syl. I decided bestow doesn't quite make sense, as Spren can't really affect the world directly, and bestow gives them a body.

Numbers may need tweaking, but I like this new design.

It is simple. Mechanically, it’s interesting — attack or block with the bonuses from the front face, then transform before damage for the back face bonuses.

Also captures the feel of Syl constantly changing into what Kaladin needs.

The typeline on the back half feels a bit weird -- Syl doesn't stop being a spren when she becomes a weapon, so why does the card reflect that she does? The answer is that spren is an enchantment subtype, which can only go on something if it's an enchantment. Much as I would love to make the back half a Legendary Enchantment Artifact - Spren Equipment, that simply doesn't fit. So a bit of awkwardness is necessary.

Also, since (ruleswise) Auras and Equipment use the same word "attach" to indicate which creature they are on, and based on the rules regarding Neglected Heirloom, I'm fairly certain this design works.