They seem similar in the sense of being geniuses lost to madness, but the context is what sets them apart.

Simon's story is about his interaction with the crown, he knows it's power, and what it is doing to him, it represents self sacrifice as he gave himself to the madness of the crown to save the people he cared about.

McGuckett's story is a little different. The story of Gravity Falls is about discovering the truth, and McGuckett's story is about the consequences of rejecting the truth. He couldn't accept what was going on around him and erased his memories of it again and again, slowly destroying who he was for a false sense of peace and safety