Shillelagh makes a club temporarily magical, meaning it overcomes resistances/immunities of many monsters (e.g. constructs, fiends, lycanthropes, etc). The cost is a 1st level spell slot (cast as a bonus action). But it doesn't work with Sneak Attack or several other "rider" effects.Whereas Magic Weapon turns any weapon magical, including weapons that can be used with Sneak Attack. Plus it follows the normal weapon rules, i.e. it's a Melee Weapon Attack, or Ranged Weapon Attack. The cost is a 2nd level spell slot (cast as a bonus action) and Concentration.
I think that illustrates the rules intent: allowing a magical weapon to apply Sneak Attack damage is more valuable than just a 1st level spell slot. And further, that it's worth more than just a 2nd level version of Shillelagh, hence the Concentration requirement on Magic Weapon.
Something to consider for your own balancing act.
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