Install Ubuntu and you get 10-15 years of support of your current software stack.

Install Debian and you get 3-5 years of support.

Ubuntu also invests a lot into their desktop experience while Debian is closer to stock Gnome. That's more of a preference thing, though.

Debian has also traditionally been a pain to use with proprietary software. Nvidia drivers, WiFi firmware, you name it. It got better in the past years but it's hard to shake the bad name.