Smartphone or GPS for World Traveling??

I do a lot traveling in Asia (Thailand, Vietnam, Cambodia, China, Hong Kong, Laos, Myanmar, Philippines, Indonesia, Malaysia), North, Central and South America and I'd like to start using a GPS to help me get to my destinations, mostly by public transit, walking (70%) and 30% by car. I go to cities but also to the countryside.

I have purchased the Rotweiller maps for SE Asia, and I have the Garmin maps for North America, and detailed third party maps for Panama and other countries, compatible with Garmin. I do have a Garmin Zumo 550 GPS. But I need a GPS that can operate 12 hours minimum, on one charge, and the Zumo 550 battery life is only about 2 hours.

I do use a pay-as-you-go GSM quad-band cell phone in all of these countries. Please note that not all countries have data, 3G and 4G yet, especially on pay-as-you-go. I'd prefer not to pay for maps & directions using data from a phone provider.

I am considering buying a Samsung Galaxy S2 phone or a Garmin hand held GPS like the Dakota 20.

What's the best go (cheapest and reliable) - get a smart phone or a Dakota 20 or other recommended handheld GPS?