GTX 1080 Ti vs. RTX 2070 Super
As you can see on the graph below, the older GTX Pascal was 3% faster on average at 1440p. Overall that means very similar performance on average, though we see big differences when looking at individual titles in some instances. The 1080 Ti, for example, was 16% faster in Gears 5, 15% faster in World of Tanks and 13% faster in Quake Champions and War Thunder. It was also up to 10% slower in CSGO and Wolfenstein Youngblood, 9% slower in Forza Horizon 4 and 8% slower in Rainbow Six Siege.

GTX 1080 Ti vs. Radeon 5700 XT
When compared to the RX 5700 XT, the 1080 Ti was 9% faster on average at 1440p and as this figure suggests, the GeForce GPU came out on top in significantly more titles. The outliers include a 34% victory in War Thunder and 32% in Vermintide 2, while it was also quite a bit faster in World of Tanks and Metro Exodus.

Now, should you buy the 1080 Ti? Only if the price is right. Looking at completed listings on eBay we see that GTX 1080 Ti's are currently selling for about $500 with the most affordable example priced at around $450. Honestly we were expecting them to be much cheaper.

You can buy a brand new RTX 2070 Super for $510. Saving ~$60 for a second hand card that probably has been abused a bit, a 1080 Ti doesn't seem like a smart investment. Moreover, you can snag brand new 5700 XT's for around $425 right now, so given the 1080 Ti was on average only 9% faster, paying a little more for a second hand model doesn't seem worth it. Based on today's results, we wouldn't pay more than $400 for a used GTX 1080 Ti, in fact they'd only start to tempt us around the $350 mark.