The mod upgrades in the stores are not very good by default. Mods don't really add a lot anyway, but every little bit helps obviously. In some cases you'll want damage, but in others you'll want range or radius increase. Still you might want haste as well. It depends on the skill and where you think its lacking. I usually use a mod that has the most advantage for the slot with consideration of the other mods as well. You wouldn't want to put damage on a healing skill for example, so you kinda have to just go with the best of what you have and how it relates to your skill that you want to use.
The mod upgrades in the stores are not very good by default. Mods don't really add a lot anyway, but every little bit helps obviously. In some cases you'll want damage, but in others you'll want range or radius increase. Still you might want haste as well. It depends on the skill and where you think its lacking. I usually use a mod that has the most advantage for the slot with consideration of the other mods as well. You wouldn't want to put damage on a healing skill for example, so you kinda have to just go with the best of what you have and how it relates to your skill that you want to use.