Not necessarily.
You seem to be treating that extra planet as a permanent addition to your empire that you couldn't get otherwise, but it's not. Chances are, even without the 80 extra Influence, you're still going to get that planet eventually, unless someone takes it from you. Everything is just delayed by however long it takes you to genrate those 80 influence (under the assumption that you're Influence-starved to begin with). So the "real" benefit here is getting a few months of extra resources earlier than you would have gained them, and a bit of extra pop growth during those months - which can definitely snowball (although, it also means that you need to build the colony ship earlier if you want to benefit from the extra resources), but it's not like you're just getting an extra planet permanently.
The +15% research on the other hand, are indeed accumulating until the end of the game (if you don't lose the planet), but are much slower to actually show any difference, and when they do, they still don't amount to much.
Overall I don't see a clear "Always pick this"-choice here. The only real thing that pushes me towards Influence is much faster if that it is much faster ifyou can make good use of it, and that situationally it can help you lay your hands on something that you might have otherwise not have gotten peacefully. But then again, that's highly situational.