We had a Citigo when they first came out and loved it. It was characterful, a bit of a laugh to drive and very well thought out. They aren’t powerful but they’re as refined as that class of car gets. Ours was our only car, as a second car they’d be excellent. The boot is pretty big too.
They are all the same car with different grilles, lights and bootlids. What you go for is up to you – the Citigo is the cheapest to buy but maybe looks the most boring while the Up! is the most expensive, but is the most stylish and as it has the VW the resale value is higher. The Mii sits somewhere between them in terms of resale and style.