Honestly, neither. I mean they are both cute.
My youngest is now 7 1/2 months & I bought a cute fairy costume for her from Target (to go with her 5 yo sister). I tried it on her last week & she looked so cute but she also looked uncomfortable (I was thinking of her rolling around/playing in it).
My kids were rolling over at 4 months & either sitting or near sitting (I have 3 kids a 5 yo, 3 yo & 7 1/2 mo).
My first was 6 1/2 months so I just got her a cute moo-cow that she could be in all day. My second was born in September so he was about a month and a half old & he was a pumpkin. But I thought this year I'd be different & get my baby something cute/pretty.
Needless to say, the fairy went back to Target & now she is going to be a duck.
Plus the weather here in CT can be iffy on Halloween so I need it to be warm just in case.
OK that is just my 2 cents but I bet what ever you put her in she will look beautiful.