pc680-
http://www.batteriesplus.com/p....aspx optima grp 51-
http://www.batteriesplus.com/p....aspx
Sizes are similar but the pc680 will be smaller. Difference will be in longevity and CCA. The 680 might give you problems in the winter because of the low CCA(220) whereas the optima is rated at 450 CCA. The big buy for the odyssey batteries is their incredibly high pulse cranking amperage. Pulse cranking(PC) is the first 6 seconds of starting, in which time most vehicles have ample time to start in.
So its def your choice on what you want to do.