Karen,
Check the archives on this subject - you will find numerous articles written on the Pro's & Con's of getting a "Working" visa. The general conception appears to be come for 90 days on your tourist visa, stay as long as you like & ONLY apply for your extended visa if/when you have to or when you are convinced you want to stay. If you just stay on without applying for the extended visa (& most people do - without any problems) you just pay a little extra at the imigracion when yo actually get round to leaving the country, US $10 (mas o menos) for every month you are beyond your tourist 90 day visa.
What area are you going to? Do you have a job already lined up? If not, you may find it harder than you think to get one - times they are a tight!!!
Good luck - Grahame.