The 10 inch elliptical, being concentric with a "surface" area about the same as the 8 incher or slightly less, in theory should give a slightly broader sweep and slightly tighter target separation than the 8 incher, but not by so much that you're likely to regard it as a worthwhile investment.
The Omega comes standard with the 10 inch elliptical, and users report that the 11 inch DD provides a really big improvement in performance on the Omega, pulling it into "running with the big dogs" territory. The slightly larger size helps, but most of the credit goes to the fact it's an elliptical DD and that the machine's response characteristics are tailored more toward DD's (the Omega being descended in part from the T2 in that respect).
The discrimination mode on the Delta is very similar to that on the Omega. Therefore I would expect that the 11 inch DD would provide performance enhancement comparable to that reported for the Omega. Perhaps there are some users here who have done that comparison with the Delta and can provide firsthand information from actual field use.
Another coil you might want to consider is the 5 inch round DD. Very good reputation for working in trashy areas, near playground equipment and steel fences, etc.
--Dave Johnson
Chief Designer, FTP-Fisher