shri - 5:13 am on Sep 20, 2011 (gmt 0)
>> Do most people who travel speak one by default, or both? What is the best practice here?
Most people in Mainland China / Taiwan will speak something closer to Mandarin (or Putonghua). In the southern regions, closer to the Canton / Guangdong provinces you'll find Cantonese or (Guangdonghua ) more popular as is the case here in Hong Kong.
Verbal Chinese can get far more confusing because of very strong regional accents. Shanghainese can have a hard time understanding someone from Beijing even though they're speaking the same language.
You also have to deal with regional dialects like Hokkien etc.
Life is much more simpler if you have to ask what script they can read - Simplified or Traditional.