Yup. Also, there is no exhaustiveness checking (the constants we chose to define for SumChoice(0) and SumChoice(1) mean nothing to the compiler anyway - exhaustiveness checking would have you test any possible int, since SumChoice is simply an alias for int).
On the first approach, with "error prone" you mean the tag could be incorrect, right? (or even have an impossible variant with 0 or multiple variants set).