I think using USB-C cable with a 20W charger will still be faster, but MagSafe will close the gap quite a bit on the iPhone 12 (I think it can do up to 15W).
Wireless charging (Mag Safe included) will always heat up the battery more for a given charge current than when plugging a cable into the phone. Not an issue in cool climes but big problem during the summer in hot places
With the addition of both fast charging and wireless charging to Apple's 2017 iPhone lineup, there are more ways than ever to charge your iPhone....
www.macrumors.com
Assuming the 15W MagSafe wireless fast charge will be similar to the 12W wire fast charge, it seems like it'll probably pretty close vs a USB-C / PD wired fast-charge with an 18W or better power adapter.