I actually don't think SPG is bad on the earnings side -- yes, the nominal, 3 points for Plat/4 points for Plat 75 isn't great, but I suspect the effective earn for most people is much better. I mostly have short, cheap stays (under $150/nt, and often much less), usually on a BRG. Under the current promo, my best earn was just over 4,000 points for a single night on a $70 rate (1,400 promo, 500 Plat, 2,000 BRG reward, points for spend), and my worst was 6,200 points for 5 nights on a $140/nt rate. That's 57 points per dollar for the first scenario, and 9 cents per dollar for the second. (I'm leaving out credit card points here, but I did earn those on top). On average, I earn well above 10 points per dollar spent.

With regard to redemptions, though, I think Marriott is better, largely due to the generous 1:3 exchange ratio. SPG category creep has eliminated most everyday good values. Of course redemptions can still make sense during high demand periods and at certain low category locations, but 9 out of 10 times for my travel patterns, redeeming SPG points doesn't make sense. Transferring to Marriott will often yield much better values.

So for me, it's the opposite of what the OP posits: I earn with SPG and burn with Marriott.