use the 8086k until you need to upgrade. For my money, the 8086k is a very capable cpu - plus there's the fact that the i9-9900k is currently unavailable unless you are willing to do like i did, and buy one out of UK at a somewhat elevated price ($620) - but that's still better than the scalper pricing on ebay and amazon ($999)
I am on the pre-order wait list on amazon, and on newegg - i finally got thru to a newegg CSR who was user friendly and he went and looked at the expected shipments on the 9900k - came back and said they expect another shipment of 100 units Nov 30th. There have been so few 9900k units showing up in discussions, ie from folks that actually have one in hand, and the few that did were in canada or UK.
From reports, intel is way behind (3.5 years on finalizing the 10mm die for the next gen CPUs, and they rehashed the architecture on the 14mm Coffee Lake CPU and can't satisfy the demand for the Coffee Lake refresh units. Heck, even the 9700K is unavailable, and pricing on the 8700k seems to have climbed a bit.
One aspect i haven't touched on, it also depends on your application - if for gaming, the 8086k should be right there at the 9900k's performance, and depending on the game, possibly better, as folks seem to be OCing it to 5.2GHz with success. I wanted the 9900K for video editing, i've already got an 8 core cpu (in my signature) but i can't get it stable above 4.3, but getting it from 4.0 to 4.3 showed me a reduction in render time that caught me by surprise, and the 5.0GHz of the 9900k just temps me, plus my X99 mobo has been buggy since the day i got it.
hope that helps some