Personally I would go for a 30 and use it as my daily bag but it depends on what you will use the bag for. Don't get a size you don't normally use just because it's rare.
I find 25cm impractical for a tote bag. This is what a birkin is...a tote.
The 30cm would be my choice.
Get the bag you will use and love...not just because it's "rare"....