working distance is going to the the most noticeable difference between those 2 lenses, and depending on what you plan on shooting you might want to go with the longer focal length. if you are planning on shooting insects or small animals which can scare easy, the extra distance will make life much easier for you.
i was playing with the idea of getting the 105mm from sigma, but ended up going with the 150mm instead and am in love with this lens. i am a big fan of sigma, i love the finish on the lens and their lenses feel nice and solid. if i were in your shoes id go with the 105.