Which CPU will run cooler? C2D or Core i5

No. At full load expect to i5 to draw a little more power and create a little more heat. At idle expect the i5 to use less power due to being 32nm and more advanced power saving features.

The i5 should use less power at almost all times. The reason is that the i5 includes the integrated graphics in that TDP rating, wheras the Core 2 does not. The i5 also does not need a northbridge on the motherboard. Because of this, the system power consumption of the i5 system should be lower at basically all times.

higher power consumption does always mean higher temperatures, the energy has to go somewhere, and unless you processor glows or buzzes its gotta turn into heat.

The key is that TDP represents maximum power consumption, and when comparing 2 generations of chips does not reflect average power (due to power saving as someguy mentioned)

Anyways I hope your not basing a purchasing decision on 8 Watts.

^ I would gladly welcome an additional ~8w of power consumption of the i5 chip considering its performance gain over e6600 anytime.  

FYI, my quad core Core i5-750 runs cooler than my dual core Pentium E6300 when they are both at stock. In addition, the dual core i5 is 32nm and so it should run even cooler.