OP - they will work fine with one another. Make sure they are close in clock / memory speeds because they will operate at whichever one is the slowest.
Looking back at the newegg details, it looks like the MSI card is already overclocked.
Core Clock: 680MHz
Memory Clock: 1900MHz
Core Clock: 600MHz
Memory Clock: 1800MHz
I'm guessing that's why the MSI card has the massive heat sink on it and the XFX doesn't. Based on this information, would it be best to accept the down-grading on the MSI card or should I try to replace the heat sink on the XFX and over-clock it?