Discernible difference? None. WB, UGL, Utility and LMG all the same there or there abouts. The difference is the snow lock on the utilities are a right PITA.
There are considerable differences Utility pouch is the smallest, the Water bottle pouch is deeper that is to say sticks out more, the UGL is taller and dept more than Utility but less than Water bottle. It all depends what you want to put in them, the UGL will hold my digital SLR, the Utility will not but it will hold a trangia mess tin or a spare lens, or a Nalgene bottle (just). Water bottle will presumably hold an Osprey Water Bottle.
Considerable? No. I sell them for a living. There are millimetres of difference. If you say that is considerable, then yes you are correct.