The difference is that one keyboard is made for humans who touch type, and the other keyboard is made for people who like to lift up their hand and move it every time they want to hit return... or left shift.
The ones with the shorter left shift and narrower/taller return key are just bad designs. They make keys you use frequently less accessible, and the tradeoff is that keys you rarely use are now in reach.
I've been using US layout since I was ~8. And all my other systems have the US layout. I am having such a hard time to get used to it.
I keep hitting ± when I mean ~ (why would I even need a ±/§ key on my keyboard?). I keep hitting \ when I want to hit Return. I keep hitting ~ when I want to hit shift.
Frustrating. Agreed with the "bad design" point.
It should be noted that on older Macbooks, the aluminum case has actual cutouts for keys. So it's not possible to change the layout of keyboard without also changing the case. I discovered this when I had my personal laptop repaired, and it came back with the wrong keyboard! The shop just instinctively ordered keyboard+case of their usual layout, so I had to have the whole thing redone again (at no extra charge, just a loss of time).
When I bought my first MBP, I wasn't completely wedded to the idea of getting a Mac necessarily - but when I noticed you could order one with a US keyboard, it tipped the balance in favour.
(For any serious work, I use a USB keyboard and mouse, so in a sense it doesn't really matter. But I use the built-in keyboard often enough that I'd be annoyed being stuck with that tiny Return key...)
(This reminded me: if you go to get a battery replacement, make sure the staff member notes that it's got a US keyboard. I took mine to a UK Apple store for a battery replacement a couple of years ago, and when I went to pick it up a few days later, it had had its keyboard replaced with a UK one! The staff member was very apologetic - perhaps they correctly read my expression of speechless shock after opening it - and it got fixed, but it took a couple more days. So in the end I was without my laptop for over a week.)
US keyboards have the narrow enter key and different placements for many symbols
I do see now that the actual physical layout might be different, having large vs. small enter key and possibly additional key next to the left shift.