A few important features made me choose the 6500 as my workhorse dmm, each hobbyist have different needs but those are things to consider when making a checklist...
Working with precision shunts, the 1 ohm range is very nice.
For my high power leds projects, the 12V diode max is much more usable.
The digitizer specs are far superior on the keithley, but admitedly only useful for special cases where scopes don't have the resolution/accuracy. For me it's a very narrow use case.
Larger buffer is great for long runnning acquisition, testing something for 24h at 1 NPLC is possible.
The switcher card expansion is compatible with the old model, easy to find on ebay.
Naming things and entering values is so much better with a touchscreen.
In canada at least, the keithley is a lot less expensive than the keysight.
On the negative:
For simple functions as a plain multimeter, direct buttons are probably more usable.
10A is only in the back.
Firmware is very young and needs improvements.
Graphing crashed on me twice. Never happened again though.
Buffer management is a little confusing because of the fragmentation, I suppose it will be solved in a later firmware.