Upgrade Order for Th8 using DE?

Statistically speaking upgrading a king from level 5 - 10 results in only around a 10% increase in health and damage compared to upgrading a from level 1 hogs to level 4 hogs which will result in around 50% increase in dmg and 100% increase in health. Also mass hogs and surgical GoHo attacks are one of the more powerful 3 star ground attacks that in certain cases may even be able to 3 star TH9s.

My suggestion is to upgrade your hogs to lvl 4 and upgrade your king in TH9 where it is relatively easier to get DE.

This depends on your current situation. If hogs are not vital or needed in your attack strategy, upgrade the King first, as the King will have more health and attack damage. When the King reaches Level 10, its ability, Iron Fist, will be automatically upgraded to Level 2. The King is also vital in to be high level, the Archer Queen will work better with a more beefier King due cost (40k to get one) and insanely low health. The higher level King will also be more effective/helpful in wars and intimidating any foes.

If hogs are vital in attacks for you, then upgrade them first before the king. It is simple logic, upgrade the most important troops first before upgrading any others. Being a maxed TH8, it is a good idea to finish up your hogs though you could upgrade your king next.