Which outfit is better?

Ok so I'm going to a wedding at the end of this month. I really don't know what to wear. It's extremely casual (bride is wearing toms, groom may not even wear a tie). Bride said that she doesn't want jeans, obviously but besides that she doesn't care what we wear. It's going to be outside, so I'm guessing it will be pretty hot.

Can't decide between the dress or the romper. I already own everything in the pictures except the romper.

Just don't know if shorts is appropriate for a wedding? The groom is my husband's cousin so I want to make sure I look appropriate, but still comfy.