Currently have a class 2 shield cell bank on my viper, not at home so I can't tell you rating, but it was 20k credits. Now, from the wiki it says that they all take 5 seconds to start once activated, and various classes and ratings have different ammo amounts. Other then that is there any benefit to a higher rating? It seems the more expensive ones weigh more but I can't find any info as to difference in time to fully restore shields.