This depends on a variety of factors. It depends on how much hit rating is available on other gear, whether 19's have access to armour specialisation, it depends on race and it also depends on how snazzy the items look. They both have the same dps, yet glacial's top end is so much higher. Glacial also has more burst due to its proc which also ignore's armour. Since we will have reforging in cata it looks like hit rating will be fairly easy to find. As such I'd say that Glacial is better, however seraphs could be useful for fighting against nelfs. If you reforged half the hit to crit then you could use glacial normally with 5% cap and then switch to seraphs for whenever you need an extra 2% hit without having to change all your gear. Useful for arena me thinks.