120mm mortar shell weights  about 12kgs and range in neighbourhood of 10kms, give or take 2kms
and 105mm shell weights about 15kgs and range about 17 kms.

SO with guns you'll get more TNT on target and longer range. Also generally artillery fires tighter group that mortars. Of course that is not such a big  factor as mortars generally shorter range  to target so dispersion is about the same. Where mortars come handy is their ability fire 120mm guided munitions (like strix) which can help you take out tanks. As to my knowledege there is no similar system for artillery in 105mm league.

Also 105mm are bit oldish systems and NATO is upping to 155mm league, where you can toss a 40 kg grenade out to 30kms with standard ammo. You can do the math....

In Finland FO would also factor in the fact that mortars fire in 6 tube company and artillery fires in 18 gun batallion, so you get about three times the explosives on target in same time.

I've favored the 120mm mtr/155mm howitzer combination for a couple decades.  Specificlly the French designed RT120 whic has a bit more range than 10km.  Lighter for both tactical & stratigic transport than any 105mm howitzer, and less maintiance overhead.  The RT120 design is approximatly three decades old.  I'd expect with a bit of R& D money an even better launcher/ammo combo could be fielded rivaling any more expensive 105mm howitzer/gun.