Now, while you don't need to take an extra proficiency for a Bard to use a Songblade as an implement (it's in the item text) this does not mean a Bard can use any Sword as an Implement. Nor can a HE Bard use his Songblade to power his Dilettante Scorching Burst.

That said, take Arcane Implement Proficiency over any MC. Why? Because AIP makes your chosen Implement compatible with all of your powers. Half-Elves get a lot of mileage out of this (hey, I can now Eyebite with my Songblade!) but there's more than that.

For example, a Goliath Dragon Sorcerer can use his Goliath Weapon Training mixed with AIP to start using a Greatsword for his powers

...and while you can't use Blade Channeling to turn your Sorcerer powers into melee Greatsword attacks, you do get the +2 damage bonus and something useful to swing for OAs.