I think the Mclaren's even more of a technological milestone, and will be much less common. There are only 375 being made and all of them are sold. The 918s are not all sold yet... I think they're actually having a hard time finding buyers.
I think they both are, to a certain extent (I don't know why you think the McLaren is so much more so) although I frankly don't find either one to be anything revolutionary (other than it signaling that if the car companies would pull their heads out of certain orifices, they could develop hybrids/electric cars that actually do more than just get somewhat better MPG, and they can get rich people to subsidize development in the process).
Its easy for manufacturers to game the "sales" on limited ones like these. If I'm not mistaken McLaren basically required the P1 to be sold out before they'd actually produce it whereas Porsche had a bunch of offers to buy the 918 if they'd build it (IIRC Porsche said that it was just a concept but they received such an overwhelming amount of people saying "build that and I'll buy it" that they built test mules to see if they could actually produce it). I saw something that said Porsche had a couple thousand offers prior to it being greenlit.
Plus, never say never on stuff like this. The F40 had its production ramped up significantly due to popularity and I believe those all appreciated.
And who's to say. I won't be surprised if there's a P1S or P1R or something, basically a revised version with a couple hundred extra units produced. Or maybe they'll just come out with an all new car that just plain trounces the P1 in a few years. There's definitely things that could offer marked performance improvements (supercapacitors, possibly paired with newer ultra dense batteries to up range and electrical performance). Much like how the Enzo and Carrera GT are much less impressive now, I think we'll see some newer ones that will make these seem dated.
Personally, I like the 918's looks better, its more classic while not looking throwback. The P1 I think will age more poorly (its far from bad, but something about it has never looked that good to me), especially if McLaren dilutes things by passing its looks onto its lower cars like they've already done. Likewise, McLaren's even had grumblings of disapproval over them using the "same engine" across their ranges as people want more for that extra almost million over the MP4-412C.