I like the SDX far better, it has DSP that works and incorporated many of the features originally found in the TS-850S/AT (which I've owned since new in 1989!) but were not found in previous versions of the 950S.
You might take a look at this excellent writeup proclaiming with great specificity the differences between the 950SD and SDX. And, oh yes, I owned both and greatly preferred the SDX.
http://www.arrl.org/members-only/prodrev/pdf/pr9212.pdfWB2WIK/6