I am going to replace the Shimano Acera X derailleur on my older Trek 920 but I am confused as to which model I should chose. The gear train is Shimano 3 X 9. There are two types of derailleurs listed. One is a "downswing" and the other is a "top swing". Since I am not familiar with their terms I am at a loss. I am trying to upgrade this bike that I have had since the 90's. Any help would be greatly appreciated. Thanks.