which brick is better for external walls

One that better meets your requirements. Aerated concrete - a popular type of brick, is lighter, easier to work with and has better thermal properties, although it doesn't matter when the walls are later insulated; however, it is highly hygroscopic, meaning it absorbs moisture and I have seen construction sites where the brick does it, and the mortar does it...but it all depends on the crew. Silicates, on the other hand, are heavier but also provide better soundproofing inside, it certainly takes longer to build with them