I've been running a DD-7 for about 4 years. It won't make the ride smoother, and it won't change the RPMs in 7th compared to the Cruise-Drive's 6th.
The difference between the 2 trannys is that DD-7 has a 12% lower 1st gear than the CD-6, and Baker says theirs is built stronger, so it will handle a much stronger engine. For that reason, some shops recommend the -7 behind larger engines, such as 124 and 131 CI.
I like my -7 just fine, but for the cost of purchase and install, would I do it again? Probably not. Some people say "Well, you can sell the CD-6 and offset the cost," but then the -7 goes with the old bike when you sell/trade, unless you swap it to the new--if possible.
This thread has some info about the work-around to (try) to get the cruise control working again:
http://www.cvoharley.com/smf/index.php?topic=105703.0. Good luck with your choice.