Recheck with your fitter. Did you do well enough with the 120 g shaft or the 105 wasn’t available? That person saw you in action. None of us has.

When looking at shafts you you should never assume that the number in the shaft name is the actual weight of the shaft. The DG 120 in stiff would be go from 118 grams to 111 grams after cutting and the DG 105 would go from 103 grams to 97 grams. Being fitted into the DG120 its just a guess without more details on your swing and testing whether the DG 105 would be suitable for you- I would trust your fitting.

Hi Tim! Weight is a tricky thing from player to player. The weight of the shaft can be one of the most useful tools in a fitter’s arsenal to influence timing of the swing sequence. Like swing weight, though, some are more sensitive than others to changes.
The nice thing with these two shafts is that they’re trimmed in an identical way, so the weight numbers will change, but the weight progression will be about the same. If you can find a demo iron with a 105 shaft and it yields the same results as what you got from the 120, then it stands to reason that they will work across the board.
I wish I could give you a yay or nay without sending you back to the demo cart, but at the end of the day we’re comparing a shaft that you hit and were fit into and a shaft that was not part of the fitting at all, so it’s all theoretical until you actually take a swing with a 105

Hi,
Never judge a shaft on paper only. The weight is as important as the flex.
My advice is: If you do not want to go and see a fitter who will analyse all the proper datas, at least you should hit and compare both shafts side by side. You should feel the difference and see the trajectory of the ball.
Personally, I am playing a modus 105 and previously I was playing a modus 120. When I was younger, I was playing true temper s300. These shafts have specific characteristics and feel.
Your swing will dictate which one is the best for you.
Fitting is the best way to start.