HGSS are good, and as they cover two regions as well as containing some 3rd and 4th Gen Pokemon, they are definitely more expansive that DPPt. 

Black and White do look good and seem to be a good rebirth of the series, and oddly I can find it cheaper to buy than HGSS around where I am. Consider the way the developers have tried to emulate Red and Blue by only having 5th Gen Pokemon until after you beat the game. If you want that experience again go for BW, but HGSS shouldnt be a waste of money either.

In my opinion the best choice it's HGSS i found them having more content than DP/Pt. But if you don't like the retro feeling of HGSS, Platinum is a great choice, Diamond/Pearl live in the shadow of Platinum.

I just asked a friend the same question and he said just go ahead and get Black/White.

I haven't played a Pokemon game since Diamond/Pearl and I was only meh but I really really want to give my DS a reason for existance so I got...one of the two versions.  I forget which one.

I'd say wait for Black or White in a few days. Apparently both games are the best Pokemon game made to date. It has seasons and all kinds of features the other games did not. Not to mention their are only 150-new pokemon you can't catch any others till you beat the campaing which should make the game less over whelming then being able to catch 649 of them off the bat.

Also considering you haven't been in with the series you will have missed several generations. This would give you a clean and fresh start. Not to mention the games visuals are better and its new so your experiancing it new just like the rest of us!

no! dont listen to these peeps pokemon HG/SS is way better then P/D/Platnium

id say either get HG or SS OR wait for black and white

trust me i know i got every pokemon game

Personally, I found HG/SS much, much better than D/P/Platinum, mainly because I was a huge fan of the original Gold/Silver/Crystal.

But since Black and White are coming out in just a few days, I would recommend them instead.

depends..

D/P/Pt have better late game content

HG/SS is terrible late game.

EDIT: Point; if you're into making new games after the Elite Four and such, get HG/SS.  If you're into late game stuff like collecting all 493 and making different teams for the Battle Tower/Frontier, get D/P/Pt.  Preferably Platinum because it pretty much iron out quite a few things wrong with Diamond and Pearl.

only problem with HG/SS is that the kanto region is boring and easy. I would defenitely get B/W

I would recommend HG/SS you'd probably get more playtime out of it since you get to travel to two pokemon regions, battle 16 gym leaders, and this game you can catch a large quantity of legendaries to boot!

I think I'll end up buying B/W in the end then, since you only get the classics in your first playthrough.