I think setting sprays are the silent heroes of makeup. After all the powdering and all the setting, those layers can look a bit...much. What a setting spray does is rehydrate the face and allow the layers to melt together once dry. If a client is looking a little powdery, set that powder away!
MAC Fix + is the undisputed champion of setting sprays. Most Makeup Artists have either heard of it, or used/are using it in their kits. MAC Fix+ is basically a glycerin infused water mix and its really good at melting a look together. Now Mario Badescu's Rose Hydrating Spray (MD) isn't actually marketed as a setting spray. To be fair that isn't the intent of the product but I used it on a day when my skin was incredibly dry and the DEW...the DEW was incredible!!! It made my makeup look like skin.
I've used both products on myself, but for clients it depends on the their skin. I found that Fix+ works well with skin that is normal to oily whereas Mario Badescu works better on people that have dry skin. Alternatively, if the client's goal is dewy fresh skin, regardless of their skin type, I will use the Mario Badescu and provide a disclaimer.
Another way to decide which to use is by basing your decision off of the type of foundation you use. Creamier products once set can actually benefit from a little rehydration. Fix + will set it beautifully but Mario Badescu will give the product some dew. For liquids, I found that the Mario Badescu broke down the products quicker. As liquid foundation breaks down, it naturally releases oils. This on top of MD will cause the client to have to retouch the face more often with a mattifying powder after about 2-3 hours or so.
So which product is the winner? Lol. This isn't that kind of post to be honest. both are winners based on the skin and foundation you are working with and the finish that you want. Both are absolutely amazing.