Personally, I think the spell-points mechanic is more appropriate to MTG than spell slots.
It also depends on if we want players to feel like the Iconic Planeswalkers of MTG lore (who are more sorcerers) or if we want them to feel like the MTG player. For the MTG player feel, traditional vancian casting based of wizard spellcasting would give a very "deck building" vibe. I don't however, think this is appropriate for the feel of PCs in the game, who IMO should come across feeling more like the Ionic Planeswalkers.
That said, I would suggest that we use the spell-point system and a hybrid of sorcerers and wizards. You prepare your spells, but you can cast as many of any one of them as you want, as long as you have the mana. You could spend all day casting Fireball, or cast a more nuanced set of spells, as if your want since Planeswalkers are shown to have innate sorcerous abilities, but also some ability to learn new spells in the way a Wizard does. Spells would all be assigned a color (based on the way the spell works, or it's outcome or whatever) and would be assigned a cost based on spell-level. You could integrate some of the alternate mana (such a phyrexian mana) fairly easily. Mana you don't specify to be a certain color would be "colorless". Magical items would likewise be given a "mana cost" to use them.
Certain classes would have mana restrictions, paladins for example may only be able to access 2 colors (Vengeance: Red/White. Ancients: Green/White. etc...) Classes that don't use spells would be given a monk-like ki system, less mana but your abilities cost less, and you are very limited on what you can do. Clerics would have access to mana based on their deities. Wizard schools would give specific bonuses to mana, IE: if you're an Illusionist you get +1 Blue mana to your mana pool /level, or spells that require blue mana cost 1 less.
I don't feel like MTG: D&D should feel like playing the card game, it should feel like playing D&D in the MTG settings with MTG flavor and variant rules.