At the same clocks the core 2 duo is faster! The pentiums have smaller L2 cache but there are so many versions you would have to pick the two processors in question to compare them.
L2 Cache is used to store instructions for the CPU to process. It sits in the CPU itself and it has low latency; meaning it is fast.
Instructions that cannot fit into the L2 Cache is stored in the system RAM which is much slower than the L2 Cache. It means the CPU takes a little longer to process because it has to wait to receive the instructions from RAM to the L2 Cache and finally to the CPU.