The 12" is the stock diameter, to gain the extra stall a high stall pump is used. This works fine for mild stall increases, up to about 22-2400 rpm. Anything after that and you start affecting the torque multiplication and efficiency of the converter. So to achieve a higher stall, a smaller diameter is used - the 11" is the next choice. Both of these converters use a very negative pump, meaning the blades of the pump are high stall, with gives you very good torque multiplication. So the benefits of using a 11" over a 12" would be a higher stall for a better 60', if you can get the traction. The cons are it costs more. The 11" core is harder to find, more expensive, and more work to convert over to a 4R100. Both converter are fine for a DD, I have the 11" in my 04 and really can not even tell it is there till you get on it. I hope this helps a little.
Chris
