Haven't tried P21s, but I have a dark blue metallic paint and I love the look of M26 on my paint. I didn't like Natty's very much at all. The deep, wet look of 26 is what I love. Keep in mind I use liquid 26.
P21S is great in clarity, but tends to brighten the color a bit. Megs26 will give you a darker appearance. P21S durability is not very good also. You'll get 2 weeks max in my experience.
The Natty's Blue should be a better match than the regular Natty's...it is formulated for dark paint. It may slightly darken the appearance. In the past I've gotten 4-6 weeks with no issues. P21S really would make the flake pop and with a 2nd coat you can get good durability.