Well, first of all I have to disagree with anyone who slags of DPPt's plot. It's decent enough - especially in Platinum where Cyrus has actually done his research and is aware of both Dialga and Palkia. Platinum is definitely one of the finest games in the series - the post-game is a massive improvement over Diamond, which before the Elite Four is still decent. The little graphical improvements and the speed boost are great too.
However, despite the nostalgia from Diamond (The last game I went into knowing nothing about, well, before LeafGreen), I do think HGSS are much better games. I love all the new graphics, particularly the grass, and the ambient noises from water etc. are good too. The improved us of the touchscreen was good as well. Plus, two whole regions!. Yes the level curve is a bit rubbish - low levels right up until Red - but on the plus side we get both Johto and Kanto.
The post-game is particularly enjoyable for an almost complete lack of evil team, tbh. You just get to get on with the main business of being a trainer - catch pokémon, battle gym leaders etc. The rocket grunt being set up for BW is good too.
There's also the little screens you get entering an area such as Mount Moon, like in FRLG.
But the real thing that clinches it for me is pokémon following you. All 493! That's the real clincher that sets it apart from all previous games and, in my eyes, any future games too - HGSS did it first.
So yeah, HGSS all the way, even though DPPt aren't bad games.