Ive used many different types of BIOS and to be honest most of them feel almost exactly the same.
The relationship is that any newer BIOS will allow you to run unsigned code.
The difference between X2/EvoX BIOX and the cromwell is that cromwell is legal, but to be honest I think even the mandrake linux website doesnt recommend running linux on your xbox lol.
The EvoX.D.6 BIOS seems to be pretty basic, I liked it, but it boots "xboxdash.xbe"-meaning you have to rename both M$ xboxdash AND your evoxdash, so every time you want to play Live you have to rename-kinda annoying, I stumbled across my EvoX2.5.colormod.err.noani.ybox BIOS not too long ago, those are cool, they change the color of the X, remove the flubber animation, give you details when you get an error, and loads yboxdash.xbe, so you can leave M$'s named xboxdash.xbe.
In my opinoin BIOS preferance is just a matter of personal customization.