Always go for local searches. IT will show you exact no. and is good for your business.

It completely depends upon your keyword. Local searches show the number of results for whatever locality has been set - most likely the U.S. if you are within the U.S. and haven't changed that locality. Global searches, as the name suggests, is the total number of world wide searches.

Some keywords would clearly be most a local search. But if you have a keyword that has international or at least multi-country appeal, and you can monetize it outside of the U.S., then global searches might matter. I say might because it depends upon where those searches are. Are they in a country that spends money? Or are they in a country that searches the Internet a lot but spends very little?

So it's not a black and white answer. You have to think about your keyword and what you're trying to accomplish and even then it is only giving you part of the answer to whether or not it's a good keyword to target.