If you want the fastest, costs are no concern alongside the power use and cooling needs(specifically on all core loads), get the i9.
If you want the fastest, and don't mind playing around with motherboard settings(helps control power use and cooling), get the i9.
If the machine makes you money, then why not the i9.
If you need something easier on the bank, the machine doesn't make you money, is more manageable on power and cooling needs, and you don't need the extra E-cores, then the i7.
Or maybe wait for the non-K chips next year?