If you are planning on getting replacement keycaps, you'll mostly only find keycaps for 1.25u bottom row. Though maybe you are ok with only changing the alphanumeric key caps.
1.5u & 1u modifiers is not an advantage to customers, but an advantage to some manufacturers I guess, because they don't have to make 1.25u molds for keys, they get away with one less key shape, and they already were making 1.5u for "TAB" and "\". There is no 1.25u elsewhere.
EDIT: Though the TAB and "\" row as different shape to the bottom row, making this point moot. I then honestly don't know why they make KB with 1.5u + 1u modifiers.
Also, the spacebar is non-standard. The central part of the keyboard has 15units of width. That means that in a standard layout, with 1.25u modifiers, the space bar is:
15 - 1.25 * 7 == 6.25u for spacebar.
On the other hand, with "non-standard" 1.5u + 1u modifiers, the spacebar is:
15 - (1.5 + 1 + 1.5 + 1.5 + 1 + 1 + 1.5) == 6u for spacebar.
I personally don't have a problem with either bottom row layout, though I haven't yet bought custom keycaps.
Cheers!
.KeyHopper.