POT has better margins and a more diversifed portfolio of businesses. Moreover, POT has investments in potash companies all over the world that throw off significant dividends whereas MOS has no such investments. MOS is still predominantly a low margin phosphate company whereas POT derives only 14% of its gross margin from phosphates. The point is that there are a lot of factors which weigh on where a stock trades and at what multiple. MOS will just sit with its cash until it is able to repurchase its shares held by the Cargill family trusts which won't happen until at least November. Until that time, MOS merely has a prettier balance sheet which is only going to get the company so far. MOS is a 5-10 year hold for those who hope to make any serious money from it. Most investors neither have the stomach or the patience for that kind of time frame.
Personally, I'll take the company with the much better balance sheet and lower P/E, especially since we know about a share buyback in November. POT is much more tied to the price of potash, which makes it riskier. Right now, POT's higher margins are translating to a higher dividend. That won't always be the case in the future. Companies with higher margins have more to lose, just ask Apple. We already know MOS wants to give back to it's shareholders. They've just been handcuffed by the Cargill agreement. I guarantee MOS will be the better investment going forward.