Is there actually a difference between the chips? 

Someone told me that the Mega8L chips were Mega 8L's that failed the 16MHz  speed tests.  I asked Atmel about this and this was their engineer's response:

Dear Customer,

All versions are basically the same device. The only difference is that they are tested with different voltage levels and speed grads. There is no requalification of failing devices from one classification to another. As you can see by the definition of the voltage level and speed grades of the AVR devices, there is a clear relation between these settings. It is therefore very likely that a specific device from one classification will work within the specification limits of another classification. But this can never be guarantied from Atmel, due to the fact that that specific device is in production only tested within its own classification limits.

Best Regards,
Atmel AVR Technical Support

So failed chips don't get sold off under a different label and it's very likely that a Mega8L will run at 16MHz  and a Mega8 will run at 2.7V.  Just note that they haven't been tested outside their specs and remember it's unlikely that either will run at 16MHz and 2.7V at the same time. 

Of course if you're buying a chip buy the one tested for the conditions you expect to use it under.  For me the decision was easy because on a Robot voltage tolerance can be an issue.  You don't want your Bot to coats to a halt in the middle of a bout because the voltage dipped below 4V.  I also don't care about the higher speed because I plan on using the internal clock which only goes up to 8MHz anyway