Just wanted to throw in my two cents on my two stays (HH Gold) -
One night at the Grand in Feb 08.
If you looking for a public transportation friendly place, this is probably not it. The walk isn't so bad down the hill vs. the Millenium, and the cabs (korean taxis are relatively cheap) and shuttle do help - but previously mentioned, there really isn't a subway station nearby. But if you are looking to party (hong-dae area) is probably the best bet and this would suit you better than the Millenium in terms of distance. Also the club/bar downstairs was SLOW when I stayed - I think I was the only customer from 11:30pm-12:30am. I think one thing that hasn't been pointed out in the forums is that the ride to the airport, be it Incheon or Gimpo is going to be about 30-45mins less than the Millenium - which in Seoul traffic could be a godsend. The rooms are pretty comfy and there is a seperate executive floor checkin area.
Three nights at the Millinuem in Apr 08.
If you litterally want to be in the middle of Seoul, this is the best place, in terms of the two hiltons (I plan on staying here in Oct 08). Seoul station is a great hub for public transportation. Plus the KTX train is there, if you want to take a trip down south. As mentioned before, the walk back up the hill from Seoul station can be a pain. Another option that hasn't been mentioned is taking Hoehyeon station (namdaemoon). It's not as direct, but the walk is a little more forgiving (Take a left out the lobby, down the hill, and stairs to the right vs. Seoul station - out and down the rear entrance/garden area). Plus the entrance to Seoul station is accross the street from the main station, and I'm a Korean-American and I feel somewhat uncomfortable in the evening times with the homeless situation in Seoul station (main lobby is ok).
Also if you decide to take a cab, it's easier for cab drivers to recognize/find the Millenium. Some drivers you say the former hotel's name for the grand. I believe it was the Swiss Grand until around 2002/3. Other than that, I find both rooms fairly comprable. The lobby area and food establishments definetely looks nicer in the Millenium. For people allergic to smoke or have young kids, I'd avoid the Millenium in the late evenings, as the entire lobby smells like smoke due to the bar situation (back in Apr 08 - i'm pretty sure things haven't changed).
I'd say you can't go wrong with either - but if you're only there for a few days, mainly on business and don't have much time to spend in the city, I'd say the Grand would probably be a better bet (especially if you have a tight schedule to and from the airport). If you want to explore and have more of the "five star" experience, go for the Millineum.