Yes and No. It all depends. If you ask this question to 100 people you will probably get 99 different answers. What works best for one may not work best for another. Its best to try as many coils as you are able and find the one that works best for you , the one you have the most confidence in.
I will say this though , even though there may be some places where that 6x9 coil really shines , many parks and old home sites are pretty heavily littered with trash and metal targets , some are really bad. The strength of the DD coils is being able to get in between those targets and separate them , singling out separate targets instead blending them together , and cutting down on target masking. And generally the smaller the DD coil is the better they are at this , though you lose a little depth. If there isn't a lot of trash and dense targets then it isn't as important , and the 6x9 concentric may do quite well , but if the targets are numerous and densely spaced then a DD coil would be your best option based on the science of how the coils work. Its good to do research and learn how the various coils work and why , this enables you to make more informed decisions on which to use in the various different environments. Then experience will teach you how to set up your detector and which coil is best to use for the particular site you want to hunt.