You see, I think the Acne Pistols were the originator of the phrase 'Mum Boot', it's the almond shaped toe and not-quite Cuban heel, and a certain roundness to the shape that calls to mind an M&S foot-glove style boot. So when everyone was going mad for them and people were desperately searching for dupes, they'd gravitate to the 'almost, but not quite', and everyone would declaim it as a 'Mum-boot'.
The phrase has now been bastardised and is trotted out everyone posts a pair of black ankle boots.
The current iteration is more angular and much more like the original Western boot (which has always been a staple for Isabel Marant, and YSL always have a version knocking about, because it's always had a hint of rock and roll).
FWIW @Hop27, I like the Millicents that you posted. I've got a pair of Aeyde Kates that are very similar, they get tons of wear (I posted them on a recent thread and they got a lot of hate!).
Additionally, I'd always get the ones you love, especially if they've been on your mind for a while. Cheaper ones will always feel a bit meh, get the the ones you want, enjoy them!