This depends on your budget for the most part, but the RTX 3080 is more powerful and will better serve for years to come whereas the 1080-Ti is already several years old.
Despite being over five years old, the GTX 1080-Ti is still a decent graphics card for gaming. Although it can’t compare to the latest high-end GPUs from Nvidia, it still delivers solid benchmark performance.
Both GPUs launched with an MSRP of $699. Thanks to the GPU shortage, it has been hard to find either card at this price until recently. As of this writing, the 1080-Ti is the more inexpensive card.