Bgaholic: USPS is best for international customers, that much I know for sure. UPS charges up the yin yang for brokerage fees and such so international customers would prefer that you ship using USPS. That being said, you may want to just look at shipping only within the US (for now) since you're just starting.
IDK about UPS for local US residents.

I've always done USPS for both local and international.
Make sure you ship WITH TRACKING for anything and everything! This is very important in case you have a buyer who is claiming "item not received". Moreover, I'd advise getting insurance to protect yourself against loss/theft of the item, esp. if the item you're selling is particularly valuable. Get signature confirmation for anything over $250.