The biggest factors are CLEANLINESS and CURE TIME with spray paint. Krylon Camo brand will hold up really freaking well if you make sure to clean whatever you're painting with a degreaser-type soap (like Fast Orange or even a grease-oriented dish soap). Then allow it to cure for a full week before touching it. I mean, let's be real, spray paint is not meant to be a permanent solution, especially for a surface that takes as much abuse as a loader shell, but it can still last for a good long time if you do it right.
I'd be interested to see what the plastidip looks like. I've seen people do whole cars in it and it actually looks ok. Not to mention it's pretty cool to just be able to peel it off and start over if you decide to change the color.