Ok, I did s a comparison test, and understand how they work.
When MBro makes a match, it searches for another Bro, looking up first, if it finds one it places a new bro on top of that one, if it already had a second bro, it places the new bro over the second; it's not immediately above, if the first match is in the first column and the first bro on top is on the last, it will still add a bro on top of him; and if the bro on top of the match is in the top row or there are nor Bros up, it starts searching down until it completes a cicle, if no bro is found then, the new bro is placed on top of the original match. It becomes chaotic when first bro added makes another match and that new match also adds a Bro, making the crazy combos MBro does when there're many Bros.
MCross works the same, but instead of searching up, it searches left, if the original match is in the bottom row the added Cross can be on the first one if the closest Cross is there; and as well it can make chains of added Cross that make combo, afterwards, as MBro, it's disappear for a while leaving only uncomboed icons the other pokemon. Added icons can destroy disruptions.
MCross needs 21 icons to evolve, MBro needs 18. MCross can use 6 candies to get down to 15 icons, MBro can use 2 to get to 16. So it does evolve slower.
I still believe it's A-Rank material as Crowd Control activates a lot and gives good power attack.