Both 3i n mg hs its own advantage Ill try list some pros n cons (feel free to add what i missed)
Pros mg :
1. Higher exp than 3i maybe bout 40%more exp
2. Higher spawn rate at tight space for aoe kill at 3rd floor.
3. Very efficient for full pt high lvl grind
4. Lot of options spawn spots
Cons mg :
1. Maybe its double 3i hp (need clarify). I feel double time needed to kill it. 2 3i for 1mg
2. If not bish or ench. Need vaccine yet still lethargic n fear make slower kill
3. Harder to form full low pt below 70 for solo grind to gain higher exp
4. Low exp efficency for spina expense if u decide to buy service (15m ea st)
5. Higher requirement to survive mg that often result lower power as compensation
Pros 3i :
1. Hv higher value exp to hp ratio than mg
2. Very easy to form full pt of low lvl below 70
3. No need vaccine for non bishop n ench
4. Bout twice faster kill for ea mob. 2 3i get 1mg
5. High exp efficiency for spina expense if u decide to buy service (5m-8m ea st)
6. Lower requirement to survive 3i that result more power can be set for min survival requirement
Cons 3i :
1. Lower summon rate than mg
2. Limited spot choice
3. Lower exp compared to mg
Overal i personally prefer 3i more than mg. Other might hv difference opinion. Hope thats help. Hv fun n enjoy t game...