I know that both were Mauser factories pre-war, how does the quality of the rifles coming out of the Borsigwalde factory compare to that coming out of the Oberndorf factory?

Sorry, never heard of it but then again I'm certainly no Mauser history expert. Perhaps you can enlighten us further? Were any rifles marked that way, the way they were marked 'Oberndorf"?
Regards, Joe

The Kar. 98k Mausers made at the Borsigwalde factory were stamped with the S/243G, S/243, 243 and ar codes on the receiver ring. To my knowledge all were marked Mod. 98 on the left rail. In contrast, the Oberndorf Mausers were marked S/42K, S/42G, S/42, 42, svw45, and byf on the receiver ring. To my knowledge, they were similarly marked Mod. 98 on the left rail.

Qualitywise you will not find any difference between Oberndorf and Borsigwalde produced (at least so marked) K98k.

Sporterized:

Very nice. Mine is a 1939. Unfortunately mine also has some Russian armory markings since it was a capture rifle. Although the Nazi armory stamps were not ground off.