Glad you made the switch. It takes a bit of time to learn, but for modeling, as well as many other tasks, the hotkey interface of XSI is what sets it apart from the rest. A lot of attention to detail is put into the layout of the default hotkeys, where as I find other applications just throw new tools in as they come but don’t fully integrate them into the interface.
I recommend making sure that File>Pref>Tools>Camera>Wheel Zoom is set to either off or navigation only. I also recommend in the same Pref’s window, setting Interaction>Tools tab to have both sticky options turned on. These used to be defaults prior to XSI 7 and were, possibly still are, not set IMO properly in recent Softimage releases. Other than that, definitely go for the default Softimage key sets, it’s well worth the trouble if your making the full switch.