Really it's just a matter of personal preference, I actually like taking off in manual mode because it's faster and landing in atti mode because gps mode seems to keep trying to correct itself which can make it difficult when landing on smaller areas. If you're still fairly new to the phantom, I'd say keep it in gps mode all the time when taking off and landing just to keep things simple, and practice flying in atti mode when you can since it's very useful for smoother videos, faster flying in case you need to come back quick, flying through tight spots, checking wind direction and speed when higher up, etc. Manual mode is also very useful if you're brave enough to try it out as well, especially when you need to drop altitude fast or if you get caught in strong winds which can almost make you get stuck in the air.