We stayed in both this past March and liked both. But for only one night, go for Park Hyatt - it is rather an iconic property (Lost in Translation), offers really amazing views of Tokyo, has a cool gym and pool (although spa is only free for Diamonds). To get there you would take a Narita Express to Shinjuku. Shinjuku station is very complicated, but if you can find the L Tower, then PH has a free shuttle to the hotel from next to that building. While there you must check the City Metr Govt building and observation deck (free), even though the views from PH are amazing too. Walking distance from PHT is a Meiji Shrine, which would be a perfect sight for a day.

Grand Hyatt is more convenient for shopping and is also very impressive, the rooms are modern and very nice, kind of angular, very comfy. For exploring Tokyo, it is probably better located close to subway. The free spa, pool gym are very nice, as is the Club Lounge (free for diamonds, offering premium breakfast spreads, and canapes with good champagne, wine (such as Sancerre)).

For a single night, I would recommend Park Hyatt.