I reckon the decision depends upon the wear rate difference of the fronts and rears. i.e. How long will you have to wait until the fronts will be down to 3mm based on past experience and the current tread depth?
If it's a while, I'd get the CS3's and wait for the next tyre change to "upgrade" the entire set. If it's not a while, then get the best tyres you can for the rears at the minute and upgrade the fronts to the same when they need changing. That way the amount of time you will have mismatched tyres will be minimised.
The fact is, it can only be as good having different tyres on the front and rears as it can be having a matching set. Most of the time, it will probably be worse.
EDIT: Just seen your last post. In that case, get a matching set, so either all CS3's or replace all the tyres and sell your fronts as part worns.