Neither of those apps have permissions to show some things... ex. data owned by other users, some system data (virtual memory and sleep image possibly). I forget right now, but I don't believe they will show you the show you the space taken by TM local snapshots either - don't have any on my machine, so I can't test that right now. Both of those apps present to you the used space based only on the data they are permitted to measure.
WhatSize in user mode also ony shows you the same data as the above 2 apps, but also lets you see the total used space - not only that it measures. Unlike the 2 apps mentioned, it has an option to measure as admin. It is the only app of these 3 that will show you the same (or very similar usage as you will find when using the About This Mac). The only thing WhatSize does not show you is resource fork data which takes minimal space.
On my current system, usage shown by the various apps
About This Mac - 231 GB
DIX - 208 GB
Omni - 223 GB
WhatSize - 231 GB in either User or Admin mode - but may provide different folder sizes when there is a difference in the 2 modes and those folder sizes will not add up to the total depending on mode being used.
Omni and WhatSize are both superior to DIX.
Omni shows basically the same folder sizes as WhatSize in User mode.
So already, it's doing a better job than DIX.
The advantage is having an option to run in Admin mode.
(It's been a few years since I tried Daisy Disk, so I can't speak to how well that app perfroms in comparison.)
Regardless, befor you end up with a computer that will no longer boot, I strongly suggest you a) boot into Safe Mode and get rid of some data or you will end up with a machine that will not boot.
Do Not try installing WhatSize or anything else until you free up some space.
While in Safe mode you may want to turn off TM local snapshots - there are plenty of tutorials for that on the web and it will auto delete the existing snapshots when you turn it off.
You better backup your computer if you don't want to lose any of your data - immediately.