from my experience, shorter stays are more likely to be upgraded. less chance of the room being booked for the duration of your stay if your stay is only for one night.

Suites in LM BKK are catered more to the artsy grown-ups than kids (as is the rest of the hotel). I would choose the SGS - it's closer to places like McDonalds/Starbucks etc, away from strip clubs and prostitutes and boasts a swimming pool that is hard to not fall in love with (esp for kids).