The preferred shares are significantly less volatile than the common shares. For most investors, that is an advantage. The preferred shares have had great capital gains in the last two years, and there is still room to grow. The regular shares theoretically have more capital gains potential, but in practice they are more risky and more likely to fall.
As for the liquidity of the preferred shares, if I was worried about the company going bankrupt or discontinuing the dividend, I wouldn't buy either preferred or common.