The 9700k has 8 cores and a higher turbo clock. Should hit the turbo on singlethreaded work and more cores for multithreaded work. HT is workload dependent, but I don't think there's a lot of workloads where it's better than adding 33% more cores.

(Cache per core reduction might be negative though)

No. HT has at best 30% improvement. And worst is it hurts ;)

The boost clocks are higher, you're probably gonna run at this more often than not when you care about it.

There maybe some work loads where having more cores will actually care, but HT isn't all that huge if a performance increase.

Base clock is meaningless now. HT performance depends on the application (sometimes HT cores mean only a few percents of improvement or performance decrease - it depends on what the "main" core does, as they share resources). The new 9700K is not something I would upgrade to (from 8700K), but it's surely the way to go if you upgrade from an older system and you stick with Intel.