No. I would say just the opposite. The clay balls that I know of like Hydroton and hydrocorn are PH neutral whereas you may not necessarily know what's in your gravel ie. limestone causing your PH to rise. Gravel may be cheaper and also about twice the weight compared to clay. The clay balls are also very easy to work with and coarse enough to drain well and pull oxygen in as it drains. I hope that helps.
I just set up a new IBC GB and used lava rock on the bottom half and expanded clay on the top half. The clay balls make working with the plants easier. I would use them exclusively except for the cost. Could run $250-$300 to fill an IBC with Clay balls. The only disadvantage to the lava rock is they need thorough washing and they don't have as large a surface area for bacteria as clay balls but if you don't need that much for filtration why spend the extra bucks? I did sort through the lava rocks to weed out the larger ones.
I am going to use Expanded Shale. Found a place about 20 minutes away that sells it for $35/cu yd. Its going to cost me more to rent the dump truck than the media would cost. Still overall, even with the truck rental, the cost is under $200 for my two 8'x3' GB. I have heard good things about it.
I've heard about expanded shale but don't have a local supplier so the shipping makes it prohibitive.
It looks like good stuff, Jeff. Slightly heavier than hydroton, but still 1/2 the weight of gravel or stone. Once I am a little further along and get it in, I will write about it.
This isn't where I get mine, but its a good resource to read about it:
I would not use clay balls. They tend to get mucky if water levels are not precise and break down into goo. The best experience I have had was with shale and lava gravel mix
I found that a good test for media in a AP system is to get a solo cup and put the media you would like to use in it and pour white vinegar in it.
If it bubbles the it will cause you problems with your PH
If it DOES NOT bubble then your good to go and will not cause you issues...
I read that tip some where and tried it on the different gravels I had around my house. I was wanting to use some white quartz but found it bubbled like I had put Mentos in diet coke so I scraped that idea and tested the red crushed granite I had and found it to be good(No Bubbles) Filled my media beds with it and lava rock and all has been good....
Just a tip that helped me when I was starting out hope it helps....