If your primary method of points accumulation will be through credit card spend, then the SPG Amex will normally be the better of the two options. (I guess if the places you tend to charge the most at don't accept Amex cards, that might not be a good option.) As mentioned, SPG points in general are earned at a lower rate (2 pts versus 10 pts per $) and thus awards are lower. So if you have a CC that is accumulating 1 pt/$ spent on most purchases, then the lower SPG award costs would tend to make it more appealing.

For example, if you want to spend 5 nights at Disney, $40K spend (or $15K spend with a 25K sign-up bonus offering) will get you enough points for 5 nights at the Swan or Dolphin hotels (which are actually on-site for Disney stays.) For Marriott the closest hotel would probably be the Marriott World Center. It will cost 140K points for the same 5 nights. So you either need $140K spend (or $70K spend with a 70K sign-up bonus offering).

So at best you would need $15K spend for SPG or $70K spend for Marriott.

Now, I am NOT comparing the programs and saying which has better hotels, better hotel coverage, better benefits, etc. If you can add in a lot of points earning from stays, then the answer may change as your earnings may be from hotel stays more so than from CC spending. And if you can get into that 270K+ points earning range with Marriott, the Travel Package awards become available to you. And personally, I feel the Marriott TP awards are the single best hotel program awards in the industry. ^