Neither. More info needed. Wait for the reviews. Wait for the price drops.Compared to the £300 7700K
, the £320 1700X does decently, I'd call it swings and roundabouts. We have leaks which say the 8700K will come in at £360; it's a safe bet that it will be better than the 7700K and the 1700X at that price.
But the 1700X is £40 = 12.5% cheaper already, and probably will drop in price within a week or 2 of the launch. Without knowing the rest of your system and the budget, and without seeing benchmarks, who knows if a £290 1700X would be a better choice than a £360 8700K...
Possibly (probably) the 1800X will get cheaper, as it could struggle to be £430 if the 8700K comprehensively beats it. And ofc the Ryzen 2xxx series is scheduled for February, only 4 months away...
Also, and I know it's a silly question, but what is there about a 4790K that doesn't cut the mustard?
Desktop: E3-1270 v5, X11SAT-F, 32GB, RX Vega 56, 250GB BX100, 2TB Ultrastar, Xonar DGX, XL2730Z + G2420HDB
HTPC: i5-2500K, DH67GD, 6GB, GT 1030 SC, 250GB BX100, 1.5TB Barracuda, Xonar DX