Yeah it's fairly contextual. Depends on a slew of factors. Rule of thumb is you leave the word you wanna emphasize by itself, i.e.
"YOU aren't strong enough to lift this boulder."
vs
"You're NOT strong enough to lift this boulder."
Same sentence, same meaning, but one is saying you specifically lack the strength to lift the boulder, but someone else might have it, and the other is saying you're an idiot for even thinking you're strong enough to lift the boulder, with the implication that basically no one else could.
'course the actual answer is whatever flows best in the given context.