David, I don't know what kind and size of tractor you have or what size offset tiller you're talking about.
 </font><font color="blue" class="small">(  So, an offset tiller is the way to go then )</font> 
Not necessarily so, in my opinion.  The only reason I used an offset tiller was money.  I had a 40" tiller that was centered and covered my tracks on both sides when I had a 
B7100 tractor.  When I traded up to a 
B2710, I offset the tiller to cover the right tracks because the tractor was wider than the tiller and I didn't want to spend the money to buy a new tiller.  But if you don't yet have a tiller, then I'd look for one just wide enough to completely cover both tracks with it centered.
 </font><font color="blue" class="small">(  what are the pros and cons of the gear versus chain drives )</font> 
I 
think the concensus of opinion is that the gear drive is stronger and will last longer.  Chains 
can stretch or break, so I wouldn't argue the point, but I've also heard that the tensioner on the chain provides some shock cushion.  Now I don't want to get into any arguments about which is better, but personally, that would not be a deciding factor for me in buying a tiller.  The one I had was a chain drive Bush Hog brand that saw about 7 trouble free years of very severe service.
 </font><font color="blue" class="small">(  something called sideshift? Is this another term for offset? )</font> 
Yes.