Wow thanks for all of the responses, I was talking with my wife after a few of these posts and one of her concerns with a tiller is the amount of rocks and shale we have in our soil. Never have using one how sensitive are tillers to rocks, my luck I would get out there and break it the first pass ;-).
I think you will find most of that with the plow first.
If you want to go DEEP to find the shale seams a sub soiler might be worth running through, just go slow and do give up when you are stopped and your wheels spin.
A sub soiler may be a good idea for new garden plots ANYWAY if the soil has been left undisturbed for decades, just to loosen it up and start with good drainage.
I have found some NASTY rocks with my 6ft King Kutter II, but no "Tiller Killers" (YET !).
Other than chipped paint; no problem, i.e. no bent tines, bent shaft, cracked gear case, etc.
There are several threads here about the relative merits of forward vs reverse rotation tillers, no need to re-hash it all in THIS thread, but I will just say that forward rotation tillers probably suffer less when they find roots and rocks.
There is a tendency for them to climb over "buried treasure", the reverse rotation tillers are more likely to hook under it.