Looking at the hotel itself, the Grand Hilton in my view is slightly better:
rooms are bigger that at the Seoul Hilton, the staff if wonderful providing an outstanding service, the Executive lounge has a very good selection of food and drinks (also better lounge breakfast). Unfortunately they removed the computers (with free internet and printer) from the Executive Floor rooms.

However, the big disadvantage of the Grand Hilton is it's location. The hotel is a bit remote in the hills northwest of downtown next to an elevated highway. Returning to the hotel in the evening, there is traffic jam all weekdays. A free shuttle to downtown is provided by the hotel.

Due it's much more convenient and central location, I usually stay at the Millennium Hilton. The rooms are also nice -although a bit small- and they have renovated some floors in a very stylish way. I always ask (and mostly get) a room with a computer (usually a laptop) with free highspeed internet. They have two Executive floor lounges, which are mostly very crowded (if not, they just close the 19th floor lounge, so the 21st floor lounge will be packed again).

If I stay in Seoul longer, I usually change the hotel for the weekend (Millennium on weekdays, Grand on the weekend).

All in all, both hotel are very good. The Grand Hilton is always around 10,000 Won (10 USD) cheaper than the Millennium.