Two different forms of music.
While there are contemporary artist that have a cross over from R&B to hip hop (but I believe that you may be speaking about rap which is different but for arguments sake I'll call it hip hop) these are two different forms of music.
WHERE THEY ARE SIMILAR AND DIFFERENT:
R&B is a marketing name which was used to replace the name "race music" in it's hey day. The styling of music is a combination of jazz, gospel, and blues influences. For example Anita Baker, Jill Scott (despite the term neo soul- another marketing tool) Luther Vandross.
Hip Hop is composed of a rhythmic, rhyming vocal style called rapping over beats and scratching by DJ's. This form of music is derived from several earlier forms of music including African (especially with the percussion beats) disco, rock, funk and R&B. This can contribute to some of it's similarities.
While they are similar in style and the texture of the music, they are two different entities created under two different guise and have two distinct styles of conveying a message to it's listeners.
Many contemporary artist have brought some of the Hip Hop feel to the R&B arena such as R. Kelly or an Aaliyah but that would still be distinctly R&B because of the styling and structure of the song, the deliverance of the message and the rhythmic organization.