I'm glad you like your setup!
I always choose a dts track over the dd track. Dts tracks are compressed at a higher bit rate which should sound better.
In terms of the 6/7.1 over 5.1, I think it depends on the size and layout of your room. If your room is small and/or the surround speakers are near the seating position, you may not notice the extra rear channels. However if your room is large and you have a lot of room between the surrounds (and behind you), the extra channels should help fill in the hole behind you.
You can experiment with the extra channels by using a pair of your old speakers. Of course they won't match sonically, but it should give you a pretty good idea if you will gain anything.
Enjoy your new setup!
Derek
edit:
Missed a question. dts-es uses one rear speaker while dd-ex uses two rear speakers, but both speakers receive the same signal.
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