The XFX are shunned by coin miners for giving low hash rate and using low quality components. Not sure how this particular model will behave.
I've read some MSI are voltage locked.
You
have to undervolt and otherwise tweak your cards with custom bios, bin files and miner software, for most hash rate OR best efficiency (depending on how much you pay for power in your region), so perhaps those 2 cards are bad options. But then, I'm talking a bit out of my ass because I don't have them.
You are the dude that wants to get 18 cards, right? Perhaps you should order 1 XFX and 1 MSI upfront to test, and then grab the one that gives best results.
Check these 2 important threads:
Speed:
https://litecointalk.org/index.php?topic=15902 (locked now)
https://litecointalk.org/index.php?topic=14996Power efficiency:
https://bitcointalk.org/index.php?topic=395004.0