Hi Paul
I've a 63 plate 520 Tourer and my wife has a 63 plate Countryman SD All4. They are very different beasts. The Countryman is fun to drive and very stable on the motorway (pretty heavy) with lots of grunt. Not sure what pack we have but make sure you can connect your phone and play music in the pack you get, drives me mad that you can't do this on ours.
Day to day it is easy to live with, but very skittish on lumpy corners and feels you are fighting it. We live in rural lancs and roads aren't the best so fell a bit beaten if you put your foot down. We changed tyres from run flats to normal and that's helped with the ride a little.
Seats are comfy and we did the NC500 in it (with camping kit and a 10 year old) so ok space wise in the back.
Other big difference is the headlights. My 520 are rubbish and the countryman lights are nuclear, even dipped you get oncoming traffic flashing you that they are undipped...
I prefer my 520 for the creature comforts and smooth ride but might be my age (40's)
If you want any specifics let me know, but would recommend a test drive on back roads (non- A roads) to ensure you are comfortable with the skittish nature.
A