I think saying NO is a better option because it appears bold and courageous. By ignoring, the other party feels that you're rude and may feel disrespected. However, NO means you stand by a firm decision, no shying away from it.
both i'd say,t depending on circumstances . there are times when your silence is Golden as you just have to ignore the issue as its not worth your effort and energy, if you know what i mean, and there are times when you got to stand your ground and say NO as ignoring will not just pass the message across.
SPEAKING YOUR TRUTH IS SO IMPORTANT. Many choose to ignore because they hate any kind of confrontation but saying no is the way. It takes practice but you have to just learn to set boundaries and stick to it.