More developmental differences
The two boys (B1 = the blonde one; B2 = the dark-haired one) are one distinctively different developmental trajectories. B2, for example, crawls as well as holds on to the crib railing and walks sideways; B1 doesn't do either of those.
And, B1 will play "fetch" -- you place something within arm's reach, as he lies on his back, and he'll grab it and hand it up to you, and laugh when you take it and say "Thank you". But when you try it with B2, he gets upset that you took it.l
Conversely, B2 will play "catch" with you, in that if you sit facing him, he'll roll a ball back to you. B1 doesn't do this.
But the most interesting difference is one that The Lady discovered by accident today, as we were browsing the toy section: B1 expresses interest in stuffed animals -- whereas B2 recoils and cries. However, it must be the face of the animal, because when we tried a stuffed bus, B2 reached for it. But when we turned it upside-down, such that the headlights emulated eyes, and the tinted front window looked a bit like a mouth, he again expressed fright.