Actually, this is quite the tricky decision between two games! Platinum was fantastic, because the storylines became quite intense when Team Galactic travelled to Mt Coronet to create the new universe, and pretty much everything in the Distortion World was awesome. The music is one of my personal favourites, and the battles against Cyrus and Giratina are good. After the storyline ends, there's the Battle Frontier, the subquest at Stark Mountain and some extras too.
On the other hand, SoulSilver...well, it was great. Most, if not, all of the routes and cities have some beautiful music. Personal favourites include:
- Mount Silver (Outside)
- Route 47/48
- Lugia's battle theme
- Route 32
- Seafoam Islands
But the music isn't everything, and I have more great things about SoulSilver. There is Team Rocket's invasion of Goldenrod Radio Tower, which had the funny tidbit of dressing up in Rocket Uniform. The Ruins of Alph are good too. The general feel of the game reminds me of when I played the game a few years back, so there is the nostalgia in a sense. There are some awesome extras, including the Pokeathlon and, of course, the Bug Catching Contest and the Safari Zone. On top of that, you have the Pokewalker and the pokemon following you in the overworld.
And there's more! The inclusion of Kanto is great, because there is more great music in the region. The gym leaders offer a fresh challenge post-E4, and the ability to rematch all of the gym leaders from Kanto and Johto is great for grinding for Red. Plus, HGSS also have the Battle Frontier (and easier to pay move tutors!).
In conclusion, Platinum is awesome. But, as the above shows (biiiiig paragraphs), SoulSilver is arguably better imo. I think that SoulSilver edges Platinum due to the great music, neat extras and general aesthetics too.