The HD5870 CF would outperform the HD6870 CF any day. So its your call.
anadtech's benchmark results show this is not 100% true. Few games where 6870 CF beats
5870 or is behind just a little bit
. This is due to better multi-gpu scaling for 6xxx series.
For the price difference between 5870 CF and 6870 CF, the better option is 6870 CF.