Imo. I would start with both. I just know that from experience.
I have a 5 year-old in Kinder, right now. His teacher is teaching him, and his class the upper and lowercase at the same time.
But what I did since my son didn't go to Pre-K, which wasn't my choice, is I made sure he knew his full name, made sure he could recognize the numbers 1-10 by using his fingers, and could say the alphabet.
Teachers these days have a different way of teaching the kiddos. By all means, is probably the best way. I know that she has sent home papers about the proper way of writing... for instance, on the writing tablet with the line above and on the bottom, and in the middle is the dashed lines. The top is referred to as the sky, middle dashed lines are known as the fence, and the bottom line is called the ground.. Its really neat. So, the sky, fence, and ground. He has to know how to count to 100, without any help. He has to know how to write his name, the proper way. He has to know how to write each letter the right way and also the correct sounds of each letter... The for instance I gave you is the proper way, at least, the rules and guide-lines that my son has to follow from the education department.
Before Kinder, I didn't really teach him the writing part. He kinda picked it up during the summer before. If he seemed interested, then I taught him the knowledge I knew. Never did I say no! So, if your young'n wants to learn to write, so be it. Lol. Let him experience with that pencil!
Also, get the books that have color-by-number. Those are excellent. A great sorce of learning. Make him notice the colors and how the words are spelled. Then it also helps with the numbers. If the word says 2 are blue. Color the word blue, blue, for him so he notices the spelling of the word blue. I did, candy corns, or skittles.. 5+1=6 He got really interested in that one. I did cherrios. Lol. It was soo much fun, too!
Pre-K is just more like recognition, or experimenting with different learning tools for the children.
That's all I can think of right now..
Got anymore questions shoot them my way. Email me at ____@____.com, that's an easier way to get a hold of me, quick!
Hope I helped some!! Much Luck to his success!!! God Bless! =]