there is no ultimate answer for your question.
Some spells benefit more from magic level than the others. However placing these data to a very simple calculator (some formulas might be outdated). It comes as cca 6 dmg difference for SD (average), 2 dmg difference for avalanche (average) (without going too deep into)
Each spell has different coeficient of scaling with magic level. Thats why SD receives more dmg with each magic level than for example thunderstorm. You can find these formulas on tibia.wikia.com/Formulae
Note the average is always a sum of minimum and maximum divided by 2.
In addition to that there is still monster weakness and possibility of crit. Meaning that the extra 5-6 average damage for SD with high roll ,critical and monster weakness can mean even 18 damage for example.
Although note that some of the formulas might be outdated and not precise, for example I believe the explosion rune formulae is completely off.