Gallade versus Mechicham!
Both Pokemon can be important parts of a team, but it really depends on its teammates to decide which Pokemon is better for you.
Gallade's base stats are higher overall, boiling down to a total of 518 versus 410.
Gallade's Attack stat is over double Medicham's at base 125 versus 60. However, Medicham's Ability choice is very important to the effectiveness of its physical moves. Gallade was designed to be a dominantly physical attacker, while Medicham can play the role of special or physical sweeper, as its typing suggests. However, Gallade's Special Attack also beats out Medicham's at 65 versus 60. Gallade's neutrally-damaging moves will be more powerful than Medicham's. Gallade and Medicham are tied up at 80 for base Speed, making them both equally suited to running fast sets.
Neither Gallade nor Medicham have exceptionally impressive bulk. Gallade's HP is 8 points above Medicham's at 68 versus 60, but Medicham's Defense is greater at 75 versus 65. Gallade's Special Defense trumps Medicham's at an impressive 115 versus 75 (however, this high Special Defense stat is less than useful with base HP of 68). Neither Gallade nor Medicham are well-suited for resisting a particular type or tanking.
Gallade and Medicham's offensive potential depends largely on the moveset you would be using, so read up on their movepools below.
Many players appreciate Medicham for its notably diverse movepool, which makes it difficult for any Pokemon to resist Medicham. Medicham's physical movepool includes Fire Punch, Thunder Punch, Ice Punch, High Jump Kick, Drain Punch, Psyshock, Psycho Cut, and Bullet Punch. This type diversity causes most players to choose a physical Medicham. Physical Medicham makes an excellent addition to a team for type coverage, making it very popular. Special Medicham's movepool is less impressive, including Psychic, Focus Blast, and Shadow Ball. If you are choosing between these Pokemon for complete type coverage, Medicham would likely suit your team better.
Gallade's physical movepool is not quite as diverse as Medicham's, but its higher base Attack means that its moves will hit harder (if Medicham can't land a supereffective move on everything it fights). Gallade's still impressive physical movepool includes Leaf Blade, Close Combat, Night Slash, Fury Cutter, Stone Edge, Psycho Cut, and much more. Gallade's movepool is large enough to help it cover for all of its weaknesses with Stone Edge for Flying-types, Poison Jab for Fairy-types, and Night Slash for Ghost-types. Gallade's special movepool is smaller and less effective than its physical movepool, including Psychic, Thunderbolt, and Shadow Ball. Due to its smaller special base stat and smaller special movepool, a special Gallade is not recommended. If you are choosing between these Pokemon for something that hits hard and can take care of its weaknesses, Gallade is a good choice.
Medicham's standard Ability is Pure Power, which doubles its base 60 Attack stat to 120, which is almost as high as Gallade's! This makes up for its low base stat and makes it even more viable as an option for type coverage. Its Hidden Ability, Telepathy, is only useful when playing battles with more than one teammate on the field, making Pure Power a much better choice (even in a double/triple/rotation battle).
Gallade's Ability, Steadfast, causes it to get a Speed boost when it flinches, which can be useful, but only in certain situations. Its Hidden Ability, Justified, boosts its Attack when it is hit by a Dark-type move. If you play with good prediction, this can be a useful ability.
Medicham and Gallade can both make good teammates, but choose the one that suits your team best!