Before D1 started learning her letters, I was quite convinced that the proper procedure was to teach lowercase letters first. The shapes are more distinct and the letters are more common. Words written in lowercase are easier to distinguish. All perfectly logical.
Although lowercase letters are more common in the world at large, I soon discovered that letters large enough to get the attention of a toddler are almost always capitals: fallen Scrabble tiles, book titles, signs. I compromised by teaching her both together.
Then she started teaching herself to write. Ninety percent of the letters she writes are capitals. For one thing, more capitals consist entirely of straight lines, which are much easier for her to write. She recognizes lowercase letters just fine, but she only writes a few that way (mostly in writing her name).
My sister brought me some mementos she found when cleaning out the house, among which was a letter I wrote at the age of four. Sure enough, it’s in all capitals. I know I recognized lowercase letters, because I learned to read from ordinary picture books about that time.
So now D2 is learning his letters, even more randomly than his sister did. Once again, he’s picking out the capitals sooner than the lowercase. The only alternative seems to be artificially limiting his environment and learning opportunities so that he only gets exposed to information in the "correct" order.