I have both the Spy and baby Spy and I love them both dearly but they are very different bags with their own good points! The baby is obviously less bulky and great to carry on the crook of the arm, it's more a weekend bag, but it holds a ton of stuff! The regular Spy is a much more luxurious bag, which is more dressy. Both bags are FUN to wear and I can wear them with nearly all my outfits, I do feel extra special when I wear either of them!
I all the different spy styles but they are all completely different bags and really can't be compared to each other.
My personal fav is the baby spy, just because its the perfect size and style for me.
As much as I love the regular spy on other people, it's just too big for me and makes me look like I'm carrying luggage. The spy hobo is so cute too, but personally I've been more into satchels lately than shoulder bags.