I'm 18- my ortho said my underbite can be fixed without surgery because its not that big. Whats difference in end result between surgery/ just braces?

Listen to orthodont. In most cases before the orthognatic surgery, you will have braces on your teeth to move them into a new position. These braces will stay on during your surgery and be adjusted about six weeks to six month after you've had jaw surgery. Your orthodontist is given you the best advice. .
Cost and discomfort. braces will likely cost less than surgery and not require the recovery that surgery requires.
All Depends. If your 'underbite' is strickly dental, then braces will fix the dental underbite w/o affecting the facial profile. If there is a skeletal component of the underbite (large lower jaw or recessessed upper jaw), there will be no profile change with only braces.
Degree. It's a matter of degree, of severity. Mild skeletal malrelationships in adults can often be camouflaged by tooth movement alone. Moderate to severe skeletal malrelationships require both tooth and bone movement. Bone movement in a non-growing adult requires jaw surgery. Your orthodontist is probably right, but if you're concerned seek a second opinion with another qualified orthodontist.
Depends on severity. I would consider a couple of opinions, especially if surgery is being considered. Sure, the surgery may correct the under bite sooner, but are the risks worth it? Sometimes there is no other option than surgery, but i would want to know more.