whats the difference other then size of the battery?

 

Thats about it. One might be a bit more amperage.

 

Group 31s typically have alot more reserve power (more CCA, HCA etc.).
I've yet to see a group 34 battery that is even remotely as strong as my odyssey pc2150s. Think about it - if the only difference between the 2 was size, why would anyone buy a bigger, heavier, harder-to-install battery?
Bigger normally = alot more power.
We need Grizzz to chime in here. He's the battery expert.

 

goto http://www.optimabatteries.com/publish/optima/americas0/en/config/splash.html...look for yourself