Hi Nadeem,
Thank you for using the Xerox forum.
The main differences are speed and toner type.
I would suggest that you review the Xerox Support & Drivers site for complete answers to your question.
You can enter the model in the center search box and then select support.
On the right column is a product specifications link (shown below)
The 2 models support pages are linked here D125 and 120EA
You also may consider using our "Contact an experienced service technician" program at the following link Fast Track to the Expert