A 6th Level Cleric Spell Is Cheaper, And The Cleric Spell Is Arguably Better
Mordenkainen's Sword is comparable to the 2nd level Cleric spell Spiritual Weapon. It has the same range, lasts for the same amount of time, and you are allowed to move and attack with it as a bonus action every round.
In Damage, Mordenkainen's Sword Wins...Sort Of
Cast as a 6th level spell (it scales with every 2 levels above 2nd) Spiritual Weapon deals 3d8+SpellCasting Ability Modifier Damage, compared to Mordenkainen's sword, which does 3d10. Both are force damage, and Mordenkainen's sword has a higher maximum damage potential...unless the Spellcaster has a +6 as their modifier. However, Mordenkainen's sword also has a lower damage minimum - 3, compared to 3+Ability Modifier.
This is the only edge that Mordenkainen's Sword has.
Mordenkainen's Sword Is More Expensive
For every casting of Mordenkainen's sword, you must first purchase a 250gp miniature platinum sword. This item is not consumed, but is a required purchase before you can cast the spell. Spiritual Weapon has no material components at all.
Mordenkainen's Sword Takes Longer To Cast
The Spiritual Weapon spell is actually a bonus action, which means you can cast it alongside a cantrip on the same round. Mordenkainen's Sword is an Action to cast, meaning any other spell you cast must be a Bonus Action.
But by far the worst part about Mordenkainen's Sword, compared to Spiritual Weapon...
Spiritual Weapon Is Not A Concentration Spell
A Cleric who casts Spiritual Weapon does not have to concentrate on it to keep attacking with it - it just keeps going on its own, and leaves you open to cast any concentration spell you like.
Ultimately, direct comparisons between damage output and effectiveness aren't going to be perfect between Arcane and Divine casters. But the list of benefits for Mordenkainen's Sword, a level above what you'd be casting Spiritual Weapon at, are so small compared to the detriments...
You'd be better off playing a Cleric.