Hi,
We have a 6000A (6034A) since 2020 here.
It´s completely identical to the 6000B except the display(HDO6000A: 12.1" 1280x800, HDO6000B: 15.6" 1920x1080).
It is our "state of the art" scope which we use for in-depth analysis/development.
As usual in this price range, each feature has to be purchased as an option and we bought the power analysis, spectrum analyzer and serial decoder package.
There are countless other options that turn the scope into a "super scope".
It has 8 simultaneously displayable math channels, 16 different grids, intel i5 processor and min 8GB RAM, it is very fast (adjustable).
I can't think of any real disadvantages, but I can only judge that from our point of view.
I don't know what applications you need the scope for.
If I remember correctly, the user "Sighound36" has a 6000B, he could perhaps also say something about it.
We don't have Tektronix here, so I can't say anything about it, the MXO5 is brand new, so nobody here will be able to tell you anything about it.
Oh well, my recommendation is to buy the scope directly from the manufacturer, you usually get much bigger discounts or options "for free" than through normal dealers.