The only real issue for i7 is number of PCIe lanes, which we do need for Resolve multi GPU setups.
and mutli i/o cards.... i have both an AJA for Nucoda and BMD for Resolve in the same machine,
I don't think Xeons with e.g. 16 cores at 2GHz would shine much at all in Resolve. This only works well for 3D rendering etc where all cores are always used at 100%.
in my existing z820 i do see all 32 cores running at 100% under Nucoda, and iin Resolve on some processes, like cacheing i see 80% across all cores.. i know some processes in Resolve are not mutli threaded, OFX come to mind...
but end of day i run a 4k feature film with EXR grades from source caches's in either software, cacheing on NR and OFX... and run and render reliably with my existing z820... that's the marks i need to meet for anywhere near the medium term, anything more is waste
No i7 machine will match it due to the number of cards i have in the chassis, not everyone's doing long form + keeping mutiple software on the same machine + comfortable changeing, upgradeing and generaly messing about under the hood + see's an ROI for a 6k machine in a 50k suite once all is done and dusted
my build may not be optimal for others, but based on the z820 i use today i expect a z840 to;
1) meet Resolve and Nucoda's current spec's
2) flexibality to add cards as needed in the future
3) grade 4k feature flims with cleints in the room without farting
4) perform well under load (100% core useage for 26 hours straight rendering a 4k DCDM for example)
an i7 build probbaly would make #3, but will not meet #1 & 2, and how well it does at #4 is to some extent about how well the details have been thought through when it's built
i have a buisness reason to invest in another machine soonish, so for me, the z840's a no brainer, it's dead cheap, dead reliable and can do all i ask of it with no extra compromises.
if the limatations of the i7 are not of concern, and building up a refurb'd z840 is too daunting, go for the i7... a good choice and leaves mroe z840's on the market for me to chose from
but please don't take offense at Baselight, Nucoda, Resolve, Quantel, Flame on and on when they spec dual xeon's.. and don't spec's a gamer i7... there's a good reason for this, i have more faith in the engineer's recomendations at any one of thsoe company's than i do in the dude nattering away on yourtube.. and no i started watching it and gave up.... everyone's got an opnion.. in my eyes he's got some poor advice to sell you... but that's just my opnion