If you are considering the Platinum card, basically think that you are paying the extra fee for the extra benefits of the card. You can probably get a better signup bonus, but don't get it just because - well, because. I don't think it's really harder to get, it doesn't necessarily give you more spending power, it just has more benefits like airport lounge access, airline fee reimbursement, etc. They are great benefits, but only if you have use for them. It is actually a worse card for point earnings than Preferred Rewards Gold because it has no bonus categories and no threshold bonus.

As I think has been explained, the distinction between MR and MR express is card color, not biz vs. personal.

Depending on how much you plan to earn and how often you plan to redeem, you might be best off starting with Blue. Then after you accumulate a pile of points, get a one-year fee-free PRG, get the signup bonus, and transfer your points.