Seitokaichou ni Chuukoku

Both are - obviously - Yaoi and both contains ( more or less heavy ) Rape scenes. ( Thought I have to admit that Koi Suru Boukun is more.. hardcore. ^^ ) 

Both are yaoi romances featuring cute schoolboys. I think you'll also see a lot of similarities between the art styles of the two, since both feature crips lines and make heavy use of pastels. 

Both feature boys in love and offer a good balance between romance, sex, and comedy 

Both are fairly explicit and have mild non-con elements with a later development into a cute and genuine relationship.  

Both anime have strong sense of cuteness throughout the series. You can see as the characters gets closer and closer to each other as the time goes by. You WILL eventually start loving both of the protagonists. The only difference is that in Seitokaichou ni Cuukoku is shown the sexual side of the relationship between characters. Perfect anime for the ones who wants something cute and to "aw" at. 

Aside from the obvious yaoi factor, both deal with friends becoming something more, and one of the characters being raped. Also, the blondes look kind of similar. Maturity pretty much the same. 

Different stories. the style is similar and the kind of reflection of the characters too. Two yaoi with developing a very attractive and funny.