One similarity between George and Theo is that both characters perceive the reality of being "saddled" with companions who require a very high level of maintenance. Theo's brother, Jacob, has Asperger's. This makes it very challenging for Theo to assert himself in the face of such a reality. George is saddled with Lennie, whose simplicity towards being in the world is a challenge, to say the least. Both Theo and George did not ask for such a reality, but both have to deal with it in their respective narratives.
Another similarity between both characters is that their realities with their companions overshadow their own sense of autonomy in the world. Theo is insistent on his independence and rebels against the structure that Jacob so desperately needs as a result. Theo rebels because he feels the need to be free from the responsibility that is Jacob. George experiences a similar reality with Lennie. One of his refrains is that he would be so "happy" if he did not have to tend to Lennie. In both settings, Jacob and Lennie severely restrict the freedom of Theo and George.
An obvious difference would be the level of responsibility between both Theo and George. Theo does not feel that he has to take care of Jacob, going to the point of breaking the law and being rebellious without Jacob's interest at heart. Theo ends up violating one of the "house rules" in how he rebuffs the idea of looking after his brother, something that Jacob embraces and embodies. This is vastly different than George who looks out for Lennie and takes care of him throughout the narrative. George's paternal care for Lennie and Theo's lack of it towards Jacob marks one of the significant differences between both characters.