I think it depends on how and when you price things. The way I look at crossfire is:
1. I can either buy a 5850 for 300 or a 5770 for 150 now and they'll both get over 30 fps in the games I play right now.
2. Some months from now when DX11 games are more mainstream and the 5770 is falling behind I now have the option of either:
a) spending about 130 and getting a second card and performance around a 5870.
b) selling it probably around $100 and getting a 5850 or similar(probably about 250 at this point).
This is all speculation about prices, but in my experience the mainstream cards keep their resale value better(since they have less to go and larger target market). The High end cards( like the 8800GTX Ultra was 700+ in 2007) can fall in value quickly(about 100 something now) so mainstream provides more options in the future with a bit of work.
My thoughts exactly.