Morph for the following reasons: Morph can be ripped into pieces and put himself back together... Mystique can take on perminant damage.
Mystique can't morph into anything larger than her own mass. She can morph into larger male characters... but only by hollowing out her insides to spread out her own mass... : Morph has turned into a whale and held Sabretooth, Blink, Iceman, Rogue and Wildchild in his mouth to transport them. He seems to have no limit on how big he can make his mass... When he turned into a giant brick and landed on Holocaust he had added weight to him so it actually hurt Holocaust. Morph can breath underwater if he changes into something with gills. Morph can fly as he has shown on many occassions. Morph can turn into stuff such as rockets and somehow propell himself with somesort of flamming conbustion.
Mystique has the skills to kill regular people and other mutants... but she can't really hurt Morph... he has no neck or bones to break... He can be cut and put himself back together with out losing any blood... where as when Mystique gets stabbed or cut by Wolverine's claws or shot by a bullet it does major damage and she can bleed to death.
I love Mystique she is ultra cool, but Morph takes this fight... this long drawn out wise cracking fight.
Morph may be the more versatile, more powerful shapeshifter, but that's not what will determine the outcome of this fight. To quote good ol Mark Twain: "It's not the size of the dog in the fight, it's the size of the fight in the dog."
Mystique is simply more clever, more determined and just plain deadlier than Morph.