I'm HH Diamond and MR Platinum Premier. I find the two pretty similar. Guaranteed lounge, few room upgrades, lots of points, good enough service at the hotels that you shouldn't run into "bad" rooms.

There's some discussion on the Marriott board how long or whether the Starwood upgrade culture will continue. SPG Platinums (pre-merger) enjoyed pretty strong suite upgrade rates at many properties. I was SPG Plat for about 4 years and it was indeed great - lots of very strong upgrades, true 1-bedroom suites and even things that would be considered parlor suites or presidential suites. That was never a big part of Marriott culture (although I did luck into one every now and then, as I have with HH Diamond), so some people think it'll fade away at Starwoods.

If lounge is your concern, then HH Diamond should work pretty well for you. HH lounges in many parts of the world can be quite worthwhile. In the U.S. and Canada they're mostly on the lame side. In Asia, they're mostly fantastic.

Diamond is sort of a no-brainer for me: if Hilton is going to pay me to carry a credit card that includes Diamond, sure...OK, I'll take it. If two or three years down the road they start pulling benefits from that card, then I'll reevaluate - especially since I have LT status at Marriott.