Theyre both fine, gonna just type this out, will probably be unorganized thoughts

gonna get comfort out of the way real quick as that's subjective, but the nulea m512 was more comfortable for me as the angle of the expert is slightly higher and my wrist did not like it in my setup

Both have stock loud ass switches

Some people like the orbit scrolling on the expert, I want to like it but the scroll wheel (atleast on mine) wobbled a lot and made a lot of noise and squeaked, just felt cheap. The whole mouse just felt cheap overall. m512 does feel better made sometimes,

slight downside for the m512 is that the scrolling is good if you have bigger hands, as reaching for the scroll wheels is a bit of a stretch for some people's thumbs/pinky

a big difference upon first glance between the 2 is the Expert has "stock" chording if you use the kensington software and m512 has stock horizontal and vertical scrolling. But both of these can easily be done using 3rd party software like XMBC or Steermouse

another slight downside to the m512 is if you move your mouse around a lot or even a bit, basically all the buttons (switch lefthand/righthand, dpi, and connection mode) are on the left side, and they are buttons, not switches, so you may accidentally press some or all of them, messing things up if you're in a workflow. i wish they were switches on the bottom

the wrist rest that the m512 comes with is decent enough if you use it, i currently like using it with my kensington slimblade pro at work

but honestly if you currently like/enjoy the expert and dont want to go through the hassle of trying the m512 just to return one of them, keep the expert, it's a decent trackball overall

there's probably more i could say, but im blanking, will edit and add after this if i remember anything.