Which is better in the longrun? To invest $1200 once every year or $100 every month?

With 0.00 US dollar commission and account management fees in the U.S. already,

both versions do not make a very big difference anymore. every month, quarterly or

semi-annually would be my choices

Depends if there are any fixed purchasing cost of your investments. If its like 10 dollars everytime you buy, then of course 1200 a year to only pay that 10 dollars once.

If there are no entry costs, and I'm intending to dollar cost average my investment, I'd invest every month or every quarter instead.