Imbue doesn't really matter for a pure nuker. The difference in damage comes in just when you use weapon skills. For example, on my hybrid spear:
Level 65 fire imbue damage: 8700
Level 49 light imbue damage: 8400
However, when it's about weapon skills, my SSS with fire imbue hits for 8k, and with thunder imbue, it hits for around 6,5 - 7k. Big difference here.
IMO, level both ice and fire imbues. Fire has more power, but you have plenty of AoE nukes with both fire and light trees, so ice imbue really helps when grinding and the damage difference is insignificant
