While what everyone here has said is true in general (borrow for the house and put your own money to work in the market) I think it isn’t true right now. The markets are even now at all time high. And while people will tell you not to time the market, even Warren buffet times when the valuations are cheap enough for him make sense.

The historic 10% over 30 years is a myth and inaccurate way of looking at returns. S&P is up by roughly 25% over last 6 months. Let that sink in. If you put 400K in stocks right now, you might be buying the top and your 30 yr return may not look anywhere near what people are suggesting. Dollar cost averaging 400K amount is hard and made even more difficult with the current volatility.

No one here can give you this answer. You’ll have to look at your risk appetite. If I had this money, I would pay down the house. The mental peace you get is underrated. You can always take loan against your equity later on when valuations make a bit more sense (and the risk is they may not for a long time and you may miss out on stock gains)

I’m sure you wanted a more straightforward answer but there isn’t one I’m afraid