Riding Lawn Mowers

I have been trying to compare the 354x and 354xd, and the only notable difference are the trans-axles. The XD is obviously the locking differential version and it seems that the X is the Fast differential, but in my Husqvarna brochure they are both advertised with Tuff-Torq differentials. While trying to figure out the difference between the two, the service manuals on both mowers have the same trans-axle model numbers. Which are Hydro-Gear models (Hydro G7-BCBB-1XDC-1FCA). Anyone have any specifics on the differences? I'm planning on buying a new mower by next week and haven't decided if i wanted the locking diff vs. the fast diff(if that's what the X model acutally is).