HG/SS is full of minigames and mini accomplishments. It's almost like the game to iddle and play once you're done with all the games. It's full of competitions, old places, new little games, and things to keep you entertained. Whereas platinum is a long full game with 1 storyline.
So you'll spend more time in HG/SS, but in truth Platinum is actually longer since most of the time spent in HGSS is "Oh look it's that old store!" or "now for one more badge!", like disconnected fragments to keep you entertained. "Now for the pokewalker!". They tacked so many things on it in order to make it bigger then original G/S, which in part was made tacked with things to make it bigger from red/blue, that it's a LOT of tacked on stuff.
In reality it's better for you to get platinum to play, and then play HGSS to iddle on it. But since HGSS is the newest one, there will be events/more focus on it. So it's like deciding between playing a new game, or playing it's first part first for the full story.
Doing a Blind (Meaning no guides or outside game trades, I'll catch as I go), Mono-Coloured, Mono-Type, Unevolved run! :)
A Water team, blue coloured.
Silver FC: 4168 7070 0507