For some reason, I always think that Arthur is stronger than Artoria. This opinion of mine arose because Arthur's Excalibur is one rank higher than Artoria's. On the other hand, Artoria has Avalon, while Arthur does not, or at least we don't know if he has it.
Well, Arthur can see simply unlock the limiters Excalibur has (though it is somewhat limited from what I remember) while Artoria can't (from what I remember/understand, though I may be wrong)
As far as I know, Arthur needs to meet several conditions to break the seals. I haven't made much progress in reading the Prototype yet, so I'm just saying what I know.
Arthur's seals are undone on a use-by-use basis, Artoria in her... I want to say interlude throws a challenge at Ritsuka so that Ritsuka meets the requirement for her comrades in arms mst be courageous, so either her seals are undone on a longer term basis or she also undoes them use-by-use and just doesn't vocalise it.
Artoria generally clears everyone here, she has a lot less restrictions than Arthur does on using Excalibur, and has overall better showings as a servant. Arthur's much more specialized into being a Beast Killer.
Charlemagne is just, at a base level conceptually weaker than Artoria. He's not a push over, but he isn't Artoria.
Richard is easily the weakest out of this selection.
Arthur would be stronger than Artoria if it was in terms of, say, who would be better to kill Draco or another Evil of Humanity, but at a broad baseline he's worse off.
It seems to me that there are no differences in skills and that of performing better as a servant seems like a strange argument to me, I remember having read somewhere that it was said that the two (Arthur and Artoria) are practically similar in everything that concerns combat skills and raw power.
Arthur Pendragon 
Charlemagne 
Richard the Lionheart 
Artoria Pendragon 
