There should not be a comparison between Arjuna and Sudama. Each had a different place in Krishna's life.
Sudama the childhood companion
Sudama was his childhood friend. Krishna must have played with him..studied with him,ran around the trees,he must have played hide and seek also...That age is meant to enjoy such things..he must have played pranks also.
That friendship was unsullied,pure,fresh and unaffected by worldly troubles.
Sudama was fortunate enough to be the partner and classmate of that Krishna who was yet to take the mantle of the great strategist who defined the course of the future of Aryavarta.
Sudama when again met Krishna after many years ,he came across the Dwarakadhish who was widely different from the endearing prankster,his mischievous Kanha who lived with him in Gurukul.But to Krishna's credit he did everything to make Sudama not get intimidated by his aura as Krishna Vaasudev.
ARJUNA entered Krishna's life when both were in prime youth. They met after Drsupadi swayamvar and never looked back since then. The Raivataka hill and the great ocean of Dwarika witnessed the comradiare.
Their friendship was spontaneous,breezy and full of life.
Two of them spent quality time in Indraprasth after Arjun's marriage with Subhadra
They went on a pleasure trip to Yamunashores They roamed in Khandav van.
Krishna Arjun friendship blossomed due to their mutual love and concern. They both are similar in every aspect.
Even their looks also prove the fact. Both are Krishnas,Dark in complexion.Both are handsome and their charm captivated men and women alike.
Both are adventurous at heart . Proved by Khandav dahan which at that moment had its own risk factor as it involved Indra. Still they went ahead and won..
While Krishna was looked in awe by every one else,Arjuna never got unnerved by his Devine aura. In Gita he admits the fact that he treated Krishna like his friend ,cousin and called him Yadava,Sakha etc.
So while Krishna enjoyed his friendship with Sudama in his pre youth,Arjuna became his counter part,alter ego or bosom friend till the end.