Though the lack of the L2 cache may not make as much difference as you might expect, it does affect overall performance some. That said, the C300A seems to be the current favorite. A word of caution, the C300A does seem to be a little trickier to overclock. It occasionally needs more than the default 2.0 volts to be stable and often needs more than a stock CPU heatsink and fan. I've seen many more cries for help from people with an unstable C300A at 450 MHz than first-time success stories.
Most people usually achieve their goal but some just never make it, no matter what they do.
We've narrowed the field somewhat, but there are still the different stepping variations to consider. Since the stepping 2 C266 is so new, there is very little known about the success rate of the SL2Y3. It may very well be as good or better than the stepping 1 CPU's. Both the of the C300A CPU's, retail boxed SL32A and the OEM and retail boxed SL2WM, are stepping 0. Many people have reported that additional cooling helps improve the success rate of reaching 450 MHz.