Honestly, both are okay grammatically, but they’re both are very blunt. If you send either of those, the other person is definitely going to feel the sting! 'Polar opposite' sounds a little more like 'movie drama’ (it’s very intense). 'Complete opposite of the person I want' is even heavier. It sounds almost like you have a list for why they aren't good enough. If you’re looking to be a bit more natural or even just slightly softer so it doesn't turn into a negative situation you could try these other ways of saying it: Relationships: 'I just don't think we're a great match' or 'I’m not really feeling a connection.' Opportunities: 'We’re not a good fit’ or ‘After some thought, I’ve realized this role isn't quite the right fit for my career goals right now.’ Or they might say: 'We've decided to move forward with someone who aligns better with the role.'  Basically, unless you’re trying to be intentionally harsh to get someone to leave you alone, those two phrases might be too strong!

July's right — both are grammatically fine, but very blunt. I'll add one more thing that might help: Why these feel harsh: Both phrases make it about the other person. You're essentially saying, "You are everything I don't want." That's why it stings — it sounds personal, even if you don't mean it that way. What we actually say: In English, we usually soften rejection by making it about fit or circumstances — not about someone's inadequacy. Dating: "I don't think we're compatible" or "I'm not feeling the connection I'm looking for." Jobs: "This role isn't the right fit for me" or "We've decided to move forward with another candidate." ~~ The bigger picture: English has a lot of politeness layers, especially around rejection. We avoid making people feel small, even when saying no. That's not fake — it's about protecting dignity (theirs and yours). If you want to sound natural and emotionally intelligent, it's good to focus on how the message lands, not just whether the grammar is correct. Great job for picking up on that! Happy learning! Does that help?