I have the OEM hitch (it was already installed when I bought the X)... never had a problem with it and I like the little step on it to help me reach my roof rack... I don't know much about aftermarket hitches... but searching in the parts and accessories forum for hitches yields a plethora of information.... here's one to get ya started...

http://www.xterraownersclub.com/ubb/ultimatebb.php?ubb=get_topic;f=4;t=004312#000000