Edit: Please see Jags comment below, which rightly disputes some of my analysis. This further emphasizes the point that this consideration is a personal one that the individual consumer needs to weight.

A further variable to toss into this mix is that Marriott hotels (with some very limited rule bending exceptions) will not honor the Gold status that accompanies the Amex Platinum if you book through AmexTravel.

Marriott Gold provides (1) 25% bonus points for your stay; (2) a further "welcome gift" of 250/500 points depending upon the hotel; (3) late checkout, (4) better complimentary internet; and (5) a possible room upgrade.

As mia alluded to, how you intend to redeem your points will ultimately guide you here. Amex points are more versatile (more transfer partners) than Marriott points, but that's not to say the answer may not be "go for the Marriott points!". It is tough to say which is more valuable because the value of the points is dependent upon the redemption. I am incredibly lazy (I don't like hunting around to min/max my MR redemption) and I just redeem my MR into my brokerage at a rate of 1.25 cents per point, through my CS Platinum. That gives me a static value of how much MR are worth and you can try to get a fix on how much Marriott points may be worth by looking at potential award nights versus the cash rate. Or, if you're lazy like me, you can average out what credit card bloggers are saying and land at a valuation of .9 cents per point. Sticking with my laziness, I will round this up to 1 cent per point for easy math...

As an example, say you want to book a night at a W Hotel. The hotel earns 5x MR if booked through AmexTravel or 10x Marriot points and 1x MR if booked direct. If the hotel is $100/night you would earn 500 MR in the first example versus 1500 Marriot Points + 100MR in the second example. The second example also yields you all of the Gold status benefits (25% bonus earn and 250 point welcome), which is why it is 1500 and not 1000.

I tend to find late checkout to be very valuable (don't like rushing in the morning) but really the major wildcard is whether or not you land a room upgrade. I have had very mixed results with this in my 10 years with SPG/Marriott status... most of the time it is a no, sometimes a nicer view, but once or twice I have found myself in a palatial suite off the back of a standard booking.