Actually 3.5 compared to 6 isn't that big an advantage (about 8% for a win, and another 2-3 for a push), I had a long write up on this a couple weeks ago when I had a team -3.5/ML, and it moved to -6 due to some information.
I also got this game -3.5 -106 myself, and I am thinking I will hold most if not all of it, depending on what it does in the interum. Thos middles between at 4 and 5 are very few and far bewteen, certainly not worth buying off what should be perceived as an advanatgeous line, especially when another lost player is a reason for the move. Injury line moves mean a lot more to me than a steam move for apparently no reason. Even if they are sometimes overrated.