The DPSM install should only take 5 to 6 hours. MINI here charge £1770 fitted. I planned to buy it at £995 and fit it myself but they offered to fit it at £1250 (a big drop). I went along with it as it meant keeping all warranty's.
I also looked at putting in the h/k as well as this was my first option before i understood the DPSM.
The h/k is a couple of hundred pounds cheaper in parts but the labour is more complicated as it is not an upgrade to the standard stereo. It uses new loom additions to allow it to take power for the amp. It has to be connected to all the speakers and headunit in a slightly different way from standard connections.
I would imagine that MINI are correct that it is very complicated to remove the h/k but 50 odd hours is a little extreme! Especially as the h/k fitting time is around 5-6 hours.
The DPSM is different as it is an upgrade to the standard headunit and was designed to work from it. It runs straight from the headunit and uses all current speaker connections that are already in place. The only extras are that the speakers have crossovers put in line from the current speaker connections to the speakers, there is a little switch under the dash, and the amp uses its own power line straight from the battery (not the loom). In the Cooper S this is ideal as the battery is right by the amp space.
The DPSM does have more parts and components but in relation to a standard headunit it simple utilises all the current holes, wires,and fixings. The h/k involves rewiring and re routing (hence why it is factory only option usually)
From my own personal opinion having the DPSM is great. Firstly i bought the car 6 months old so i had no choice to have h/k from factory. Secondly i would have spent 600 - 800 on aftermarket anyway. I didn't want to have to have a face off stereo and i still have warranty.
I still think that factory fitted h/k is a very cheap option for what you get but if you can go for the DPSM it really is worth the money.