That size kite (the 12m) can use either bar, so, it depends which other size waroo you anticipate gettting. If you anticipate getting the 9m in the near future, then get the 45cm bar. If you anticipate getting the 16, then get the 55cm. That way you can use the same bar on both kites

Some people like to have two bars, one backup. So, size of the next kite will still matter.