While the two can be used to achieve the same objective in some cases(in this case blocking flash ads) it's not intended to that, each one complement other but one can't do everything the other can. I will explain.
"Click to play" helps because: by blocking Java, Flash, SilverLight, Shockwave, Vlc plugin and more you stop a lot of tracking around the web that happens without you to see...you think that all flash that happens on a page are videos or iterative content? No, surprise you the ones that track you are well invisible to your naked eye(you can learn more about this seeing by yourself how much plugins leak data : Here and Here and Here ) also stops things that happens automatically and annoy you like videos, audio and etc and also make pages load faster on slow connections.
Unfortunately on most browsers you can't choose what you want to allow or not, your either allow flash or java for an entire page or you block it, so to see an video you wan't you will have to allow flash on this page, the same flash that plays your video will load the ads and track you if they want but it's better than allow that on every page on internet you visit. You would be surprised about how much sites ask Flash without a visible reason at all.
Adblockers helps because : it blocks the ads that also can track you and are obviously annoying, but ads can show in many ways including flash, java, just images and etc. Some Adblockers can block specific scripts used to delivery ads(not 8 or 80 as "click to play")
Thats why each one complement each other but can't replace other. I personally use Ublock Origin since it's open source work very well and others Adblockers had a few controversy in the past like phoning home and etc...it's available for all major browsers
