DDR or DDR2 SDRAM for my new build?

Is DDR2 THAT much better than DDR?

I purchased two (2) sticks of Kingston Technologies 512MB DDR SDRAM Memory (KVR400/512R, PC3200/PC2700/PC2100) on sale for $30/stick after rebates a couple of weeks ago at CC.

Now the Kingston Technologies 512MB DDR2 SDRAM Memory (KVR533D2/512R, PC2-4200) is on sale for $35/stick after rebates.

My question: Is there enough difference in DDR and DDR2 that I should return the DDR and buy the DDR2? I'm buying hardware as it goes no sale so I'm in no real big hurry.

I've not opened the first memory and can return it easily.

DDR and DDR2 are not compatible; you have to buy whatever your motherboard supports, not what's cheaper/better.