The Core 2 Duo is more advantage with regards to clock-speed (3GHz) over Core 2 Quad (2.4GHz) and bus frequency making this 'robust' in many applications..Hence, if speed is issue then go for Core 2 Duo..
On the other hand, Core 2 Quad 'literally' uses 4 processors hence, this makes them 'powerful' in many 'Monster' applications and since it uses 4 processors then there's no need to say 'literally' that the cache will be bigger than core 2 duo.. but the 'price-to-pay' of course is the heat dissipation for this processor..
However, kung gagamitin lang ung computer for internet, emails, office applications (word, excel, powerpoint..), etc.. then i guess P4 can do the job.. 'maximum usage at less cost'
Most of computers with Windows Vista OS are using Core 2's processors.. pero kung ako tatanungin ok na 'ko sa winXP SP2 or SP3 with P4's..
In my opinion, ndi naman kasi lahat ng 'power ng CPU' ay nagagamit in most of our applications..