Both are bittersweet stories about smart and hard-working female rulers (reigning queen Seonduk and dowager queen Chunchu) who loved their kingdom most of all. Both have lots of scheming and little romance (particulary little in QS)

Both are fantasy epics with gorgeous high-budget production and beautiful visuals.
Both center on warfare and politics, with lots of manipulations and bloodshed. Both are stories of few young characters who are destined to play important roles in the history.
Both have no logical endings (

Both are typical time travel stories about a young female time traveller and her boyfriend in a fictional ancient kingdom.

So, I recommend Shin Don only because it has common historic characters with Faith. You'd watch it if you have educational purposes (want to learn more about the period).

Both dramas are the stories of the blessed true king’s rise to power facing many hard trials when he/she goes up from level to level, but sometimes pays high price for it.
Of course, these two dramas have big difference: First King’s Four Gods (aka Legend) is a fantasy with lots of magic, Queen Seonduk is just a historic biopic (So, FKFG is about king Arthur, QSD is about Elisabeth I Tudor). FKFG was shortened and got sudden ending (with lost plotlines), QSD was prolonged and got additional story arc in its final part (thrilling but hard to watch).
Though QSD is a political drama almostly cleaned from supernatural it has many mythological tropes (like sky omens, prophecies etc.). QSD takes some plot twists and typical chacracters from FKFG or from other previous K-dramas about great kings.

Also in both dramas the unification of various Korean kingdoms is a sacred super-goal that justifies any means

I wouldn’t say: if you like QSD then you'll like EK. I’d say: if you feel that QSD lacks smth, watch EK and vice versa.

In 2009 QSD had madly high rates at the Korean TV.
That’s why in 2013 the EK writers took the QSD plot pattern as the success formula: a smart strong female lead goes up to the royal power, first she cross-dresses as a boy, then fights against an evil chancellor. There are some male leads for who she is a love interest: one is virtuous and very pragmatic, another one has unhealthy psyche. There is the third character (very cute) but there are no romance between them, alas.
QSD has a lot of complicated political affairs, mind games, multidimensional characters but it lacks romance, pathos, adventures.
In EK writers added all this and made a more vivid teenage version of the same plot. Also they made the characters not so complicated and made the female protagonist even supersmart and superstrong.

Both are rather standard Chinese romantic costume dramas with beautiful costumes, fighting kingdomes, mad love, Mary Sue female lead and male lead called 'great warlord' (though it is not clear why he is great). If you are a shipper you'd like both

Secrets of Three Kingdoms (as the story of the sibling substitution) is actually an alternate history version of the famous classic plot of the Romance of Three Kingdoms . They have many same characters but Secrets consider how history would be different.

Both are costume dramas about intrigues, ambitions and scheming between princes at the royal court where people look for power, justice and revenge. Both have typical Chinese ending (I'd call it 'Chinese nationhood wins'). Both have beautiful visuals and very good acting. Rise of Phoenixes is a little bit more slow but has a real female lead and more romance.

Both series have partly same historical characters. But the biopic Jeong Do Jeon is not so explosive.

In both costume dramas you can find a politician hero played by brilliant Zhao Lixin and many other ancient Chinese policemakers and warlords.

Crazy Queen looks like Go Princess Go but it is worse. The time-traveller in Crazy Queen is a lady of a male harem. But dialogues are not so amusing and the plot is more stupid and dragged.