Hi Meltem both of them have the same mean.

سلام ، من دانشجوي دكتري حسابداري هستم و در حال آموزش زبان اينگليسي ، خوشحال ميشم با هم آشنا بشيم

Both of them have the same meaning; but in detail, I can say that "khodahafez" is combination of "God" in Persian with "Protective" in Arabic. and about "khodanegahdar" both of the words "khoda+negahdar" are Persian with the same meaning.

there are still other words that mean goodbye in persian. just like farewell, so long, ... بدرود، در پناه خدا(الله) و عبارت هایی دیگر

no dif at all . it is just like hi and hello