Hi - I can tell by your question that you already have a good grasp of these two. Outstanding - 99% of the time is a positive expression. 1% is maybe if someone is being sarcastic. But you can think of it as a positive. "That was an outstanding meal!" "That was an outstanding fireworks display!" Stands out - Anything that calls attention and memory. Can be positive or negative. If something "stood out" to you, it sticks in your brain. "Her outfit stands out, because it is a rainbow pattern." "He always stood out to me, because he was so rude."

Outstanding has > 1 definition so it's essentially > 1 word. It's sometimes used in the sense of "remarkably good". But it's also very often used to mean "unresolved", as in "outstanding debts" or "outstanding issues". Unresolved is, in my opinion, by definition a negative thing. There might be other meanings. "To stand out" means to be very conspicuous. There's no implication in the word of +ve or -ve. By the way you can also turn "to stand out" into a one-word modifier, as in "Led Zeppelin gave a standout performance last night". This form tends to be used in positive contexts.