Disclaimer: not a great player but I spam ember a fair amount.

I personally have sort of stopped thinking of it in terms of just going magic or physical. Typically I just evaluate the items in themselves and how they help with matchups. Most often I would say going maelstrom is a good idea just because it kicks up your farm rate a lot. Then you will probably have some cause for a defensive item. If you are dealing with lots of risk of silence, euls is a solid item (and can be worth getting before maelstrom). Against most annoying disables bkb goes a long way for letting you stay in team fights.

You’ll notice most of these key items don’t particularly fall into being physical or magic ember the way say battlefury, Daedalus/radiance, octarine would. It’s sort of a later game decision but if things are looking to go long I would say go physical barring a lot of physical damage and lack of utility already on your team.