In order to keep costs reasonable I am going with the traditional white weather balloon. It will be a 4' (capable) balloon inflated to just over 3' diameter. I shouldn't have to worry about bursting as I will not be near max inflation and I bought a quality product -just like the pro's use. The trade off for strength is weight, I have to balance the two. Too strong and it's too heavy. Helium isn't cheap. One concern I have is that temperatures might be in the 20's (Fahrenheit) and that means I won't have as much expansion. I suppose that might be offset by the fact that the ambient air will also be more dense and so it will not be a factor. The physicists here will know.
I am trying to avoid (burst balloon) prop entanglement by having the securing cord run through a plastic tube for 15" before attaching to the P3P. That way, in the event of bursting, the balloon would pivot downwards but be further out than the props. I can't make this a fail-safe system and the craft may well torque over if that happened. My best bet is to avoid it. Regardless, no human life aboard so no need to worry to excess. Sometimes if you think about things too much you kinda freeze up.
I am not too worried about getting blown away because I will only fly in no-to-low wind conditions, and not exceed 400' altitude over a 10 acre open area. I have a very nice German side cocker pellet rifle, but I'll leave that at home. The idea makes me smile, though the thought of having to shoot my baby down seems hard to take. I'd rather it just kept going. Bon voyage! Shouldn't be an issue I hope.