well Hunza has its own charm with Rakaposhi view, apple orchards, Karimaabad n aliabad villages, Minapin village, Baltit n altit fort and out class hotels.
On the other side, Neelum is just awesome with its deep forests, deep silence, less population and relatively less tourism activities.
Less number of tourist go to Neelum as compared to Shogran and Hunza specially in March...
If you like loneliness then you should choose Neelum. You can explore leepa Valley, Pir chinasi (2 extra ordinary places) Muzzafarabad, Jamun Kashmir Border with Neelum. Firstly you will go to Muzzafarabad. mean to say Muzzafarabad will be your base camp. Leepa valley is on one side and neelum on other side of Muzzafarabad.
on the other side, hunza has all year tourism activities with all time road open. and has all type of hotels...
now depends upon your taste, which you choose, neelum or hunza.
Shogran is open in March and most possibly Siri pay are accessible in march. because snow falling ration varies every year...