The reflections causing problems with your sensors thing is not too common BUT not too rare either, there is no way to predict how your sensors will react to the light refraction. The only thing you can do is just watch closely and be ready to do...
I have a mini 3P and mini 4P. The 3 never gets flown, the 4 a lot. I've kept the 3 as a backup.
If I get a 5P, what's the argument for keeping the 3P?
I'm not coming up with anything.
I keep my drones, I figure I will never get near the money I have spent on them anyways. and after you strip the electronics to feed the other Drone, simply paint the old plastic body silver and you have a nifty flagpole topper!
I'm keeping all of my drones for 2 main reasons mostly due to the unknown whether they will ever be widely available in the future: 1. If one of my drones breaks down and I can't get a repair or replacement, I'll have a suitable backup and 2...