I am craving a bull pup with a low powered variable for a general purpose rifle. Do either the Tavor or X95 feature ergonomics in which one would be better than the other with a low powered variable optic?

The ergos are better on the X95 in general, but I don't think there is anything specific that makes it better with a low powered optic.  I think the x95 is better with an optic in general because the top of the buttpad lines up with the top of the receiver, while the Tavor butstock has that weird drop (although that can be fixed with the Manticore Arms replacement buttpad.

i'm not seeing how what optic you use will make one rifle's ergonomics better than another

Cheek weld I suppose. It's not to critical with a dot. It's hard to tell shouldering the rifles without any thing mounted at the gun shop

I have a m21 on my sar21 and have no issues.
I just  installed a manicore curved but pad and it is even more comfortable to shoot. With that said i had no issues with the stock butpad it is just way more comfortable with  the curved pad.

I prefer the SAR to the X95.  Get your hands on both.  It's personal preference.

Im going on my Experience only, the X95 Feels smaller and better suited for that type of Optic, the balance is slightly different.

However, my Tavor Shoots much better its easily a 1.5 MOA gun. Sometimes with handloads and a good day I can do 1 MOA groups.

My X95 is about a 2.5 MOA gun sometimes 2 If I have a good day and match ammo.

Is this accuracy difference a big deal ? Depends on your intended use, these guns were designed for Close Quarters Urban fighting and thats where they excel, anything more than 250 Yards Id rather have a AR with a free float rail and decent barrel.

This.  Perhaps it is the results of many more rounds on the SAR.   It just feels right as compared to my X95.  Either way, you will need an ADM Recon H mount too get the proper cheek weld.