Battle Frontier
1. Multiple game modes (Single, Double, Multi)
2. Multiple facilities with different rulesets
3. Forced to Set mode
4. Forced to level 50 or 100, putting every trainer on the same range
5. Enemies are randomized; can't know in advance what the opponents will use
6. Must succeed multiple battles in a row
7. Many banned pokemon
8. Only available in Platinum

Post-game battles
1. Singles only
2. Regular battles only
3. Shift mode available
4. No forced level, can just be 10 or even 20 levels above opponent
5. No randomization, can always know in advance what the opponent will use. Most opponents use mono-type teams allowing for easy stomping. Boring to fight them multiple time since they'll always use the same team.
6. Can be fought one at a time
7. Any pokemon can be used, including Mewtwo, Mew, Jirachi, Palkia, Dalgia and Giratina
8. A romhack will add those teams to Platinum within a few weeks

Remember, Platinum ALWAYS win.