My S9+ is working perfectly, no lag, and is even faster and smoother than both the U11+ and the Pixel 2 XL.
Yes, battery will be better in the U11+ of course, because there's a huge difference in terms of size (3500mAh vs 4000mAh), but so far I'm achieving the same battery consumptions that I had with my previous Pixel 2 XL, but with the difference of having AOD enable. I suspect I can get even more battery if I disable AOD.
I'm pretty happy with the device, and is my first Samsung ever and I don't regret it at all; is true that so many options can be overwhelming, but is just a matter of configure properly each of those and the phone will have an stellar performance.
;)