Lets consider that a FULL INT has 296 INT Points and 13 Points into CON on level 280 with 30 bonus points, and that a FULL CON has 309 points into CON on level 280 with 30 bonus points.
Lets also consider both have the following end game gear:
2 * Wiz Bracelet = 64 Matk
Animus = 86 Matk
Enchanted Headgear = 120 Matk
Circle Rod + 7 = 384 Matk
Arde Dagger = 153 elemental
4 * lVl 6 blue gems slotted = 100 Matk
which gives us
892 + 64 + 86 + 120 + 384 + 100 = 1646matk + 153 elemental for FULL INT
284 + 64 + 86 + 120 + 384 + 100 = 1038matk + 153 elemental for FULL CON
Considering a lvl 280 monster (same level as the character) has around ~480 Mdef, and considering max enhance with quickcast, simplified dmg formula would be:
(Base Skill Dmg + Matk*SkillModifier - Mdef + EleDmg) * Quick Cast attr * Enhance * Number of Hits
So FULL INT Frost Cloud lvl 5 would be:
(643 + 1643*1 - 480 + 153) * 1.5 * 2 * 32 = 188064
FULL CON Frost Cloud lvl 5 would be:
(643 + 1038*1 - 480 + 153) * 1.5 * 2 *32 = 129984
Frost Cloud FULL INT/FULL CON = 188064/129984 = 144.68%
FULL INT Meteor lvl 10 would be:
(3229 + 1643*4 - 480 + 153) * 1.5 * 2 * 1 = 28422
FULL CON Meteor lvl 10 would be:
(3229 + 1038*4 - 480 + 153) * 1.5 * 2 * 1 = 21162
Meteor FULL INT/FULL CON = 28422/21162 = 134.30%
FULL INT Freezing Sphere lvl 10 would be:
(943 + 1643*1 - 480 + 153) * 1.5 * 2 * 1 = 6777
FULL CON Freezing Sphere lvl 10 would be:
(943 + 1038*1 - 480 + 153) * 1.5 * 2 * 1 = 4962
Freezing Sphere FULL INT/FULL CON = 6777/4962 = 136.57%
So, from these numbers alone, its obvious that higher INT helps a lot with dmg, specially from skills that:
- Multi hits
- Do not have high base dmg
Hail for example is one skill that scales greatly with Matk for the above reasons (did not calculate Hail damage because its pretty hard to hit all hits, so its damage is very inconsistent)
so a 34~45% increase in damage in not neglectable, however, you do lose a ton of hp (a FULL INT with good gear would be around 15k, while a FULL CON would be at 48k on level 280). So you trade 34~45% in damage for 70% in hp.