The boot lock is on all three of the variants, not just the Mii, although only the Up has the all glass tailgate like a C1/107/Aygo. Maybe it is a cynical link to the Fox ;-) but I suspect it's merely simpler and cheaper to fit that than the
badge mech as we are used to in larger Seats and VWs.
Personally, I like the look of the Up better and it has the more stylish rear quarter window shape that the Mii and Citigo don't.
As requested a bit further up, the 1.2 TSI would make for a nippy model. However, the Arosa
Cupra/Racer never saw the light of day, so I will not be holding my breath for a more powerful version of the Mii. I'm sure someone will do a conversion sooner rather than later. The Up GT concept 'has' a 98hp 1.0 unit, that might be the best we offically get.