On a dual channel platform, running dual channel is optimal in terms of performance. Its not like you will see the difference in real time operations, but there will be some difference in scaling and performance.
12gb vs 16gb depends on your workload. Ofcourse 12gb is allright but having some headroom doesnt hurt. Like some games nowadays consumes around 12gb and your pc needs some additional RAM to run the background operations. Then again, the 2500k is already a pretty old cpu and spending on 16gb RAM on it is a pretty tentative decision, specially since RAM prices are pretty high currently.