I'm leveling my ice wizard and not sure what will be the best second line for it, this char will be mostly used in keep siege

None of the spells in Earth or Fire specs are great at low level, but there are arguments to be made for both lines. In Fire, you don't get any useful new spells, but you will reduce the variance on your baseline fire bolt's damage. In Earth, you get a 26 seconds AoE root, which, due to resists, will be a very short duration, it could still be worth it.

For keep sieges, I would go for the Fire line. You'll be using that bolt very often, and your single target Ice root should be sufficient for other purposes.