My baby was swaddled arms down - which transition product should I buy?

You can try either! The SWADDLE UP™ TRANSITION BAG (previously called SWADDLE UP™ 50/50), is a '3-in-1', converting from Swaddle to Transition Bag to Sleep Bag once both wings are off.
The SWADDLE UP™ TRANSITION SUIT is a '5-in-1' solution that can be used as a Swaddle Suit, Transition Suit, Sleep Suit, Play Suit and Travel Suit all in ONE.