fix+ and prep and prime skin do entirely different things.
fix+ is absolutely amazing, and is a staple in my everyday makeup routine in a variety of ways. spray a couple sprays on your powder brush, then run it though loose blot powder to set your makeup. sweep your 187 through your msf, then spray the brush with fix+ and apply to cheeks for an extra glow, or spray it directly onto your face, from about a foot away, to kind of "set" your makeup, if you will. it also smells lovely, and is great to have in your purse or car for a pick-me-up while on the go. plus, a bottle lasts for freaking ever.
prep and prime goes UNDER your makeup, and just helps it go on smoother, and stay longer. smashbox makes a similar face primer, which, IMO, is better. still, both the primer and fix+ are essentials, to me.