We have and use all three DMMs, 2 KS34465A, DMM6500 and SDM3065X.
The SDM3065X is the weakest of the 3, it doesn't have many of the features of the others and doesn't seem in the same league, something mentioned about the long history of KS (heritage with the HP34401A) and Keithley with DMMs seems to be present in their usage. However the SDM3065X also is ~1/2 the cost, so from a value standpoint it's quite good.
The KS34465A just does everything well, and is very easy to use and quickly get to what's important.....accurate measurements!! Being older still prefer the buttons to menus of the DMM6500.
The DMM6500 excels in the graphics area, almost like having a low sweep rate scope. Also has the lower ohms ranges, but doesn't have the lower current range of the KS. When long term measurements are required we turn to the DMM6500.
Both the KS and DMM6500 have switchable rear inputs, we've found these handy and hang thermistors from and record local ambient temperature without having to remove leads from the front inputs. The KS uses a pre-aged/selected LM399 reference, believe the DMM also uses a pre-aged reference but don't think the SDM3065X uses a pre-aged LM399.
We purchased the first KS34465A a couple years ago, then the SDM3065X, then the DMM6500 about 6 months ago. When we needed another quality DMM a month ago, it was between the KS and DMM, and finally decided on the KS.
Edit: The cap measurements are OK, more accurate on the KS and DMM. However, consider a dedicated LCR meter like the DE-5000, much more accurate, versatile and useful in every respect.
Best,