Do not have either of the models you mention, but have a 14 year old 646SC Plus that has never broken down, only requires salt every 11 months,(live in a soft water area) and still cleans as well as the day I installed it. Love the top rack for cutlery, so convenient.
Best suggestion, buy the more basic model, you will probably only use two programs at the most, ie the economy and an occasional universal plus. Any other programs, unless you have specific reason for them are a bit of a waste of time imho.
For the money Miele represent very good value and a dish washer is a very simple appliance, just make sure it is as full as possible for it to save money on water and electricity.
Hope that helps, Philip.