I do not think Pakistan and India are looking to gain anything practical from joining the NSG, both already have access to civilian nuclear technology/materials from China and the USA respectively.
It seems to be an issue of "prestige" for India (as is common for them). And if they join the NSG they will be able to veto Pakistan's future entry, which is why Pakistan is trying to join now, to prevent that.
China's position is that India cannot gain NSG membership unless Pakistan does as well. And we have already vetoed India's previous attempt to get in.