You could have both, but i see no reason to. I use ATM its manager instead of killer. It works well. I paid for the ad free version. It works well when i need it. I use it mostly to kill tasks when games start to skip frames or lag and occasionally when an app hangs.
I just use Android to kill tasks when I need to ( very very rarely. ) Less apps is better, and task killers count as apps. (Obviously I'm not saying don't download anything. Just saying I can do everything task killers can do, so I'm not going to download any. More resources for everything else. )