As you have noticed there doesn't seem to be a big difference in terms of listed benefits between a Marriott Gold and Plat. At the lower end properties, there aren't too many benefits that they can hand out, so there is really very little difference. At the FS properties, since Golds get CL access and upgrades if available, there may not seem to be a lot of difference there either. So as you mention the real benefit seems to be the 48 hr guarantee and possibly the arrival gift or 500 pts.

My suggestion is if you want the nights to satisfy the Megabonus promo, then stay the needed nights at Marriott. If you don't, but there are promos in one or more of the other chains that you can maximize, you might be better off looking there instead. Attaining Plat status over Gold would be down on my list of reasons for changing from one chain to another.