It is very hard to say, but you should certainly stick with only one programme. Generally UA will be more generous in earning and spending (miles never expire) but you will need to figure out if you are likely to achieve status with the spending requirements. SQ miles expire after three years, and they can be harder to use. M&M would be another option if you live in Italy (using LH, LX, or OS), but you'd need to see if you will qualify for any status. Short answer: unless you fly significantly more than 25,000 miles and not in the cheapest economy classes, it probably doesn't matter much where you park you miles.