I value SPG Plat above HH Diamond (I also have both). I too am pretty annoyed about the effective removal of Diamond force and the monkeying around with inventory to pretty much take out capacity at popular properties at popular times of the year.

I spend less time in the US than I do in Europe and Asia so the US lounges don't bother me too much but from what I have seen of US Hilton lounges they aren't really oases of free pouring alcoholic drinks. I tend to think that Starwood properties are better on the whole. When suite upgrades are given (rarely in my experience these days compared to frequently as a new SPG Plat in 2006) they are better but the worst outcome at a Starwood property is a non-suite upgrade (or no upgrade at all) plus lounge access is the same as the best upgrade (per official policy) at Hilton.

SPG Plat gives you 500 SPG points per stay, Hilton gives you free breakfast if there is no lounge. The big difference though is redemption availability with no manipulation of inventory. That is the reason why I won't make any effort to renew HH Diamond in 2012 (my Diamond expires in 2013).