I would argue that the similarity between these two characters, who at first glance seem to be profoundly different, lies in their relation to other characters in the play, most importantly to Iago and Desdemona.
Clearly what seems to link these two characters is the way that both are used and manipulated by Iago for his own ends. We see Roderigo being manipulated in this way by Iago in Act I scene 1 as Iago gets him to alert Brabantio to his daughter's elopement with Othello. Similarly, in Act II scene 3, we see Iago tempting Cassio to get drunk so that he can provoke an argument between Cassio and Roderigo, and get Cassio into trouble. Both characters are therefore alike in the way that they are used by Iago for his own nefarious purposes.
In addition, there does seem to be a similarity in the way that both characters are in love or at least attracted by Desdemona. Roderigo makes no secret of his love for Brabantio's daughter, and his jealousy is something that is easily exploited by Othello. We never have a definite answer as to whether Cassio is in love with Desdemona, but it is certainly his attraction and proximity to her that is exploited, again by Iago, to bring about his revenge.