Seems like HG/SS are better to play than D/P/Pl. Although both seem to very good.
You're right. I hadn't done much research into the games. I just wasn't sure where to start. Sounds like all five had their strong points. Though more people are raving about HG/SS more so than D/P/Pl.
Google the game and usually somewhere on the top there should be a link to the bulbapedia which has the best descriptions imo.
Also avoid Diamond and Pearl, they are much slower and have some issues compared to platinum. Honestly, the only reason why I'd get them is to fill up the pokedex (unless you have previous game like Emerald or FireRed from which you can transfer Pokemon) or to get Darkrai and Shaymin through a glitch AKA tweaking.
I won't go into details but both games have good stuff which the other one doesn't. Platinum has Pokeradar and Poketch. Story is amazing, a bit short for my taste, Distortion world is ABSOLUTELY flawless. Music is probably my favorite out of all the games I've played from main series (or at least somewhere on the top), and places are memorable, they are not random places made just to fill up the region.
Soulsilver on the other hand has something unbeatable and it is a following Pokemon (Platinum has that too, but only in one park), it also has two regions, a great(!) story because Team Rocket is the best villain team ever, an a*s rival you won't feel sorry beating, music is amazing too. Sadly it's missing poketch and pokeradar but you can eventually buy both games to experience all of it.
So honestly? Buy both, they aren't that expensive and you can get them cheap. In case you are planning to game the rest of the games, Black/White/2/X/Y, play Platinum then Soulsilver/HG. That's how I did it. I played all the games that way I could intertwine nostalgia and new stuff. If you played in Kanto or Johto before, Soulsilver might be a bit dull as you will know what happens. Platinum should be "fresh" enough, so by the time you're finished, Soulsilver will seem fresh enough and you might even forget some story parts, then Black/White and so on.
Ofc, the only downside of this is that after Platinum you will be used to the new graphics, so this huge step between Crystal/Firered and Soulsilver/HG wont be so stunning.
There, my answer stays the same as before, I just added some details. If you want new stuff, go with platinum first, if you want childhood memories, go with SS/HG first.
My ultimate advice is to play it slowly and enjoy the game :)
Pce