Seriously? The 8700 has 6 cores and 12 threads, so it has way more power...
It is a bit faster per core, and has SMT allowing it to better handle more than 6 threads, but it's also way more expensive. Looking at US online pricing, the i5-9400F is currently around $155, while the i7-8700 is $300 or more, effectively doubling the cost of the upgrade. And the vast majority of current games won't even fully utilize those extra threads.
In terms of clock rates, the 8100 is clocked at 3.6Ghz with four cores, while the 9400 is clocked at 4.1GHz with 6 cores, and the 8700 adds Hyperthreading and can boost between 4.6 to 4.3Ghz depending on the number of active cores, assuming it has proper cooling. The 8700 is probably a bit more "future-proof", but I'm not sure that justifies it costing around $150 more.
As for whether even the 9400F is a worthwhile upgrade right now, if you can sell the 8100 to recoup a good chunk of its purchase price it might be worthwhile. I would not expect a big difference in the performance of most of today's games though. With a 1070 Ti running at 1080p resolution, moving from an i3-8100 all the way to an i7-8700 is only likely to boost average frame rates by around 10% or so in most games, with the 9400F performing a little behind that. And unless you have a high refresh rate screen that can display those extra frames, you probably won't notice much difference in most current titles. And if you are running a higher resolution like 1440p, the performance difference between these processors in current games will be even smaller. The extra cores might still help smooth performance and avoid stutters in some situations though, and that will likely become more common in future games that utilize more cores.