Skill build is Game dependant and build dependant,:
Overall your Flame Guard is your most valuable skill, aoe damage and magic damage denial.
Sleight of Hand is always maxed second independant of the situation, even in spell build, the damage is just too much.
It is possible to place your skill points without skilling either guard or chains, if the game i just that dependant on that skill or that independant from the other skill, works either way.

Phys build is recommended for both positions 1 and 2, it spikes when you get the Crystalys component of your Daedalus so go agressive there.

Spell build is recommended for position 2, mostly if you're not the win condition for your team, it offers a more bulky position 2 and more sustained damage, for team fight setup win conditions such as an enigma team or a magnus team that is dependant on their ultimates always max E first. Doesn't have any major spikes, but after drums you should have enough levels on your spells to go nuts, all depends on the game here.

The skill build will be my recommended Default for Phys Ember, pretty much how i always run position 1 against the typical supports for my low low rating (lion and things of the like).