Shimano Road BB cups are 1mm wider than MTB, normally on 68mm road frame they are used without spacers, you can use MTB cups but I’d put 1mm either side. Note MTB BB also comes with a wider tube between cups which you can’t use on a road frame without deforming it (or using all the spacers) they are generally meant for 73mm BBs.
The XT BB has a bigger bearing than the Deore BB, and seem better sealed. I went through a pile of Deore BBs before I started using XT. An XT BB is only £10 on CRC (cheaper than the Deore!) so I haven’t tried the XTR as XT works for me.
The thread on a Mountain BB is longer on the drive side in case you are using a BB mounted derailleur. The Saint BB has an even longer thread for BB mounted chain device. The Saint BB looks otherwise idential to the XT.
GXP uses a stepped crank (24mm/22mm) I’ve tried using shims on a Shimano BB (24mm) but had problems with chainring offset and eventually binned the GXP crank. GXP BBs don’t seem to last either, and are limited in choice, which was the main reason I changed to Shimano.