There are two points in the game where you have to choose one item over another: The pyramid ruins, and the Rainbow Shell. Everyone seems to suggest taking the Safe Helm from the ruins and the Prism Dress from the Rainbow Shell. My question is: why? Especially for the Prism Dress. There are 3 other Prism Dresses in the game (on the first playthrough): Run the Black Omen in 1000 AD, make sure you take Ayla with you, and charm it from Zeal. Pop in the Epoch, head to 600 AD, and do it again. Go back further, to 12000 BC, and run it a third time. Bingo, three Prism Dresses. And, lo and behold, you have three girls on your team. You will also have charmed away 3 Prism Helms this way. Now, you have 3 Prism Dresses, and 3 Prism Helms. Why would you want to get a fourth Prism Dress, which no one can use, instead of three more of one of the game's most powerful helms?
As for the Swallow, since you can also charm a Safe Helm away from the Lavos Spawn in the Black Omen, I'd pick up the weapon, since it is the only way to obtain the Swallow in the game, and I'm a perfectionist.