The SDT-750 is more complicated and does more. I've been developing a list that compares the two and will post that with more editing.
The HX9V is easier for my 65 year old brain to become comfortable with.
As I've said before, I have trouble keeping track of what came from which camera after I've shot it, renamed the file and loaded it on my media player.
There are frustrations with the HX9V. For example, at a football game yesterday, the HX9V was slow to get going. I had to make sure each clip started early and, this morning, I trimmed what I didn't want in the easy to use Sony PMB software.
The SDT-750 is quicker to respond, has a "pre record" function that keeps 3 seconds of video BEFORE you press the button and has a viewfinder. It was a little easier taking football video with it until the the guard told me "video taping" is against the rules.
I'm finding I grab the Sony first most of the time. It is easy to fit in my pocket, it is less obvious to the people around me and the output is very good.
If both were stolen, I would replace the HX9V first. Then, maybe, I would start saving for the yet to be released or announced TM1000 or equivalent.