Well Magnums are better in softer terain, but not by much. I think that the 589s are probaly better for you because of your familiarity with them and there over all performance. If I were explaining this in a percentage stand point I would have to say that if you are doing more than 50% of your riding on soft terain 589s are probaly better performace wise, but if you get into the 60-70% range then the Magnums would probaly be better for you.

For the better ride, lower cost (most of the time), and lose of weight (very slight amount-- but everylittle bit counts) I think the 589s are the better choice for you from my experiences in the with you.