We need some better questions, this is too confusing. Anyway, I'll give it a go: presuming that you mean Fenris-Levi and Cain-Lilith, then it is the addition of the LRed main cannon on the Lilith, while the Fenris effectively only gets a hull strength upgrade to become the Leviathan. Yes, there are other subtle weapon loadout differences but that's the big one. Of course, if you are really comparing the different ships, then the terran cruisers each have 2 AAAf while the Shivan ones only have 1 each. Otherwise the change from FS1-era cruiser to FS2-era is generally equivalent from Terran to Shivan.