25mm or 30mm tube?

Optically, not very much: Resale value, then you'll probably sell a 30mm tube a bit quicker in the U.K.
 
Assuming the internal mechanisms for windage and elevation are made to take advantage of the increased space then they will allow more adjustments hence the huge range of adjustment in the newer Zeiss and other 34mm scopes. Also from an engineering point of view a 30mm tube will be slightly stronger than a 1" tube of the same weight.
 
The 30mm has more advantages all round, these are better light transmission lens quality and coatings being equal. more windage and elevation.It is easier to align. the larger diameter lens during manufacture. contoured lens are better if they have a reduced angle due to a larger diameter. a thin walled tube of a larger diameter is stronger than a thin walled tube of a smaller diameter. For some reasons the Americans seem to think a 1" is still better this is usually due to ignorant gun writers in the magazines and a reluctance to accept that anything from outside the states can be better,superior or invented any where other than the states
 
Will it really, can you explain that one?

For a given length of tube you'll get greater strength by increasing the diameter of the pipe work more than by the thickness of the metal. In a scope the result would be marginal at best but still a selling point and possibly helps with the warranty if it is any stronger. In reality the 30mm gives more space inside the tube for the mechanicals and allows for more windage and elevation and also easier/cheaper/quicker fitting of the working parts inside too.
 
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