Better to buy ETFs all at once or gradually dip in?

I'd like to invest £10k in VTI / VOO. I was going to buy all in in equal portions between the two. But a friend told me it's better to invest a smaller amount every month (like 1k) over a longer period (10 months).

I don't know which of the two I should do, and why one is better than the other (because the ETFs on long-term will always go up, right?).