I will quote myself

Quote Originally Posted by drewhemm View Post
...has anyone been able to go much higher than 441/3.3Ghz with the non power saving E5420 variant?
Just did some searching on this topic and on Google and it seems like people struggle to get stable with the E5420 past 2.8-3.0Ghz so it does appear as though the L5420 has more headroom - rock solid stable at 3.3GHz...

It wouldn't surprise me if the L5420s are machined to a higher quality than the E5420s, that would explain their better thermal efficiency, better overclocking potential and of course the 25% higher price point!