The more expensive model has the new C3 stepping while the less expensive model has the C2 stepping.
CPU stepping is soft of like a revision by the manufacturer. Later stepping are usually desired because the manufacturer has refined the chip, tweaked it a bit more, adjusted voltage, and/or fixed some bugs. Later stepping chips sometimes overclock better.
If you plan to overclock, then you might want to pay for the C3 stepping. Otherwise, you wouldn't notice the difference.
The model number listed on the Biostar website is the processor SKU, while the retail box parts have a separate SKU. According to CPU World, the 925 SKU that Biostar is listing is the newer stepping. In short, both old and new stepping 925 processors will work just fine on your motherboard. Be sure to update the BIOS to the latest revision before upgrading though.