Sorry, but there is a *big* difference: Whiskey Lake 'fixes' Meltdown - thus does not need either KVA shadow nor L1TF flush - mitigations that can have *big* impact in some workloads. Similar to Coffeelake-R.
Unfortunately like Skylake/Kabylake, Whiskey it is not able to use Retpoline (at least the standard version) thus IBRS/IBPB are needed for Spectre 2 kernel transitions. As Haswell/Broadwell (and earlier) can use Retpoline they may end up faster in some workloads than Skylake/Kabylake/Coffeelake - a ridiculous state of affairs.
I'd say only buy Whiskey Lake and *not* any of the older ones unless they are massively discounted. If buying now at all.
You can feel the responsiveness of the system with the mitigations on/off so the older systems 'feel' sluggish even on raw compute workloads you may not see a difference. But if you benchmark responsiveness that involve user/kernel treansitions the difference can be large.