Given the opportunity to make a pre-seed investment in common versus preferred shares, should the default be to buy the preferred shares

As a VC, you must get preferred shares. This is sort of non-negotiable. Common is significantly less valuable because it has no preference. Unless the founder will do all future financings via common, which is highly unlikely and creates future financing risk for the company because future VCs will likely not invest, you need to ensure that your investment either converts to preferred shares via a SAFE or Convertible note, or directly acquires preferred shares.