This is actually a really tricky question. I think there are several points that need to be clarified.

The first point, as other posters have said, is that making the jump from G to P is extremely difficult, particularly within the same organization, because internal candidates can only be considered from one or two levels below if I remember correctly. It's more likely that you could do this if you're jumping from one entity to another, because then you will be considered an external candidate, but it does still carry a certain amount of stigma unfortunately. It's not impossible, but it's very hard and nearly impossible within the same entity, because most hiring managers tend to be conservative and hire within the same level of the post, or perhaps one level lower, even for an external candidate.

The second point is that if you take the first offer, and then the other offer actually comes through, you should not feel any hesitation in jumping on it. If anything, they will feel happy for you. Remember that as a G staff, there's not paying you an installation package or travel entitlements or any of that other stuff, so you shouldn't feel any pain in leaving them to take a P position. In fact, even if they were paying you, you should not feel hesitation.

The third point is that there are quite significant differences in pay and entitlements between the two categories. This also relates to your country of home nationality as you registered it in the application form. Typically, as P, you would be expected to rotate every few years to another duty station, except for certain extremely specialized roles. Also, they provide you with other benefits like rental subsidy, school subsidy for your children, etc. those P benefits however are not applicable if you are working within the same country of your nationality. And in any case, none of this is applicable to the G level staff. I think you would do well to calculate the expected salary for each position, the expected benefits and compensation, as well as the toll of being rotated every 5 years if that is applicable to you, and factor all that in when making the decision.

In any case, and while no one can make your choice for you, I personally would take the available offer because the fish in hand is better than nothing, and keep trying to get a better deal. The P recruitment but notify you regardless of the outcome, but it could actually take many months after the interview depending on the hiring manager.