Platinum is way better.
There are many differences.
-Move Tutors in 3 different locations. They teach good moves.
-Battleground-rebattle gymleaders that have high level pokemon. (Good for training if you want level 100s)
-Rival's pokemon get up to level 80.
-You can get both Dialga and Palkia in this game. (Along w/ Giratina.)
-You can get Eevee earlier on.
-You get a Porygon in Veilstone.
-Less berries are needed to exchange for contest items.
-Battle Frontier. (It's better than the one in Hoenn, imo.)
-Surf is faster.
-Better graphics.
-Battles against Cheryl, Mira, Riley, Marley, and Buck.
-More Pokemon can be obtained earlier: Mamoswine, Gliscor, Dusknoir, Yanmega, Lickilicky, Rhyperior, PorygonZ, Tangrowth, Probopass, Togekiss.
-Articuno, Zapdos and Moltres are now in this game.
-Obtain a Togepi egg from Cynthia.
-Wifi plaza
-Distortion world
-You can get either Armor Fossil or Skull fossil in the underground depending on whether or not your ID number is odd or even.
Here's more on Serebii:
https://www.serebii.net/platinum/changes.shtml
Hope I helped. :)