Performance and thermal efficiency are not terribly different.
The 8600k tends to have better gaming performance by a narrow margin but Ryzen slightly outperforms Coffee lake on productivity applications or games where there is very good multi-thread support. Knowing exactly what you will mainly use this for will largely determine which platform works best for you.
Honestly, if you're looking at buying now you'll probably want to go with Ryzen since those are actually available. The Intel products are not currently in stock anywhere and there's no telling when they will be or for how long once they are.