I would do Alpha.
Last year my ODS did Primer (he turned 5 in November) and he did it really easily. He loved it though, having his work sheet. He will be doing Alpha this year and will be turning 6. It really does review what Primer introduces. My DD started with Alpha, no Primer, and did great.
Primer is not necessary, but is nice if you have a younger one that likes to have their “own” work.