These two reels are built for different things. The Revo Rocket is basically a Premier with the only difference being that a Premier is lighter due to a carbon side plate but the Rocket has a faster gear ratio. The Rocket is also built on the newer Gen 3 design which is MUCH more comfortable to hold in my opinion. The MGX is built to cast light baits but the reel itself is bulkier due to the different design. MGX is lighter than the Rocket though.
Long story short, if you want a reel that is designed to throw light baits, go with the MGX. If you want a reel with fast retrieve that would be good for topwaters, flipping/pitching, or anything else where it is important to take in line quickly, go with the Rocket.