Get a heavy box frame disk and it will cut even in tough clay. You can make many passes with a disk in the same time it takes to make one with a tiller. You can even add concrete blocks or railroad steel (if you can find them) for extra weight on the disk. I have rocky soil with rocks varying from 3/4" to 12" or larger and would never try to run a tiller in it prior to removing the large rocks. Even then it may bind up with a missed rock. I have been using an 8 foot disk on my garden but now I want to get a tiller so I can thoroughly mix in leaves for added organic matter. I have disked it and picked up most of the larger rocks and still found several to locked up my walk behind tiller. I plan to get a 54" to use behind my B26 this fall and hopefully wont find too many more rocks since I have already used the disk to loosen it up 8" or so in depth.
I would advise you to do the same, buy a heavy disk to use first, pick up all the exposed rocks then use the tiller to break up any clods and thoroughly mix the soil, but not so much that is it fluffed up like flour.
