Whether you have the regular Galaxy S8 or S9, or even the larger Plus models, you'll still find that the Galaxy S10 and S10 Plus have extra lenses. With the Galaxy S10, you get a main camera, a zoomed lens, and an ultra-wide-angle lens, which is great and gives you way more versatility in what kinds of photos you can shoot.
At the end of the day, however, the main and most important camera on both the Galaxy S8 and S9 are still great shooters.
As for selfie cameras, only the $1,000 Galaxy S10 Plus comes with a dual-lens selfie camera, which offers a slightly wider-angle selfie compared to the regular selfie camera. The second selfie camera on the S10 Plus also supposedly helps for better portrait mode shots, but the S8, S9, and regular S10 can do portrait mode photos just fine.