In an situation where the couple can not get married at present, I need advice on which one of the two is more appropriate and preferable. I understand that there is no concept of mangni in Islam; nevertheless, it is an accepted custom in Pakistan. Nikah is of course the Islamic way of doing it.
Mangni can be broken. Nikah requires an actual talaaq to break it.
If you both want that there should be no allowance for changing situation/feelings due to time and distance, do nikah -- but remember: you're married now.
If you want to announce your serious intent to marry, but want to make sure you can survive time and distance first, do mangni -- but remember: gossipers and meddlesome aunties can mess things up while the other person is away. And remember also that living abroad will change a person. The person remaining at home will not necessarily change with the traveller.
What is Mangni ? I do not know what is that , we do not have it here in our county . I can guss this is a relation between couples but till what extend can you please explain to me Mangani?