Yes and no.
Here is the golden rule: In crossfire, all the cards run at the speed of the slower card.
So YES, there is a disadvantage to pair 2 cards not the same speed. So technically, an ASUS 5870 paired with an XFX 5870 would "downgrade" the ASUS model to run at the same speed than the XFX one. The ASUS 5870 is clocked higher than XFX.
However, NO you won't see any real impact in this particular case. The reason? Except the ASUS 5870 I don't know any 5870 that aren't a sticker over the reference design. What I mean is that all the companies seem to have just put their sticker on the AMD/ATI card, they have all the same clock speed, the same fan, the same everyting. Based on that, current 5870 are all the same, so there won't be any impact using another brand. Even the ASUS 5870 is only a slightly overclock "same thing"...
Personnaly, if I had an XFX, I probably want another XFX just for the sake of having to deal with a single company if anything happens. The fact that XFX offers a lifetime warranty would encourage me further to get a second XFX.
If you got the money and can't wait, buy another brand, it doesn't really matter in THAT particular case.