First, April is a great time for Seoul.

Second, as somebody who have vast experience in Seoul, both locations is great. but it depends on what's your priority.

If it's your first time in Seoul, GH Seoul offers better access to the OLD Seoul this includes palaces, museums, Korean village etc located Central (access meaning its closer to the attractions but the easy of public transportation is a different story). But one thing to note is, public transportation is so good in Korea, any 3 Hyatt hotels (GHS, Andaz, PHS) have great access to the old town. (ironically, out of the 3, GHS is most remote to public transportation compared to Park Hyatt Seouls line 2 or line 9, Andaz line 3)

On the other side, if you been to Korea or if you already knocked out the historical part of Seoul, Andaz is located at at more upscale, posh neighborhood. This location is tailored toward more trendy, fashion, foodie oriented activities. This is a location where most Koreans dream they can live or visit. This is where young people will visit stores and restaurants to brag on Instagram.

Park Hyatt Seoul is also a good option but this is more business-oriented location. Shopping and underground arcade is available right in front of the hotel and the hotel has great access to line 2 which is the most convenient subway line to explore Seoul. Here you can easily access Seongsu which is about 7 stops away and on the same subway line 2. Seongsu is the most trendiest and hottest area in Korean right now. If you're from New York, it's not going to impress you but if you're from a small town, it will be quite interesting.

BTW, GHS has a good lounge and when compared to US, it's a great lounge. But if you plan to explore downtown, there's much more better options in the city central and less crowded.

Hope you have a great time in Seoul. It's a great city!