The advantage of MagSafe 2 over the original MagSafe is simply its slimness: It’s easier to fit the new, slimmer port into very thin Mac laptops.
The MacBook Air uses
a 45W MagSafe 2 adapter; the MacBook Pro with Retina display uses an
85W MagSafe 2 adapter. If history is a guide, you can mix and match those MagSafe 2 adapters, but the lower-powered one will take longer to charge the MacBook Pro. (The 85W one can’t charge a MacBook Air any faster, either.)