It would be a no brainer if you were only looking at the player's ability but I'll bet Cissokho will want at least 10k a week more than Pieters as well as the fee being higher. If that doesn't get the hierarchy running scared I don't know what will.
Sorry if it was a bit unclear, what I meant was that if all the monetary factors (wages, fees etc.) were held constant and we were looking at ability alone, then Cissokho should definitely be the one to pursue. We're talking about a player who was linked to Barcelona not too long ago and one that I thought would be 'too good' to want to sign for us.
If he wants to join us and is willing to accept a sensible wage (even if it would be higher than what Pieters would accept), then we should be busting a gut to get him through the door. 8 million euros for him would be a steal.