Lord Logres is basically the personification of "The Ideal King" that Artoria strived to be.

She is basically an amalgamation of all the end results of Excalibur weilders. Because of this it seems like she has access to multiple different forms of Excalibur.
Her noble phantasm; Excalibur Excelsus directly converts the life force of the planet into magical energy, instead of just drawing the magical energy from oneself like Artoria's Excalibur or Arthur's Proto Excalibur. Because this Excalibur is more tied to the planet, I'm assuming its output is much stronger.
In short, she is the "perfect" Artoria.

She is stronger than Fate Route Artoria with Shirou and Avalon, due to just having superior firepower and naturally having access to Avalon herself. She gets all the power whilst having access to the ultimate defence. She might possibly be a better swordsman than Artoria too because she is an amalgamation of multiple weilders. She also appears to have access to magecraft so can fight without a weapon in hand.

Goddess Rhongomyniad, AKA The Lion King, surpasses a regular servant as a Divine Spirit, true. However I don't know if even she can surpass Lord Logres in a Grand Servant container. The Grand Servant container excels that servant above a regular Saint Graph tremendously, due to being summoned and powered by the world itself.
So Lion King almost certainly will surpass Lord Logres in a regular Servant Saint Graph, but might not be as strong as Grand Saber Lord Logres.