Nitro X
I would have to agree with Robie that 9.3/74 cases and ammo are easier to find than 375 flanged mag.
That said, even though I have a couple of hundred new Norma 9.3/74 cases in my cupboard, I'd go for the 375 flanged mag as it can easily do anything the 9.3 can do and at a lower chamber pressure.
If at some later time the rifle has to be resold, the 375 will draw a lot more interest than the 9.3 (except maybe in Europe) and should either be easier to sell or will bring better dollars or both.
As far as pushing veloity goes, this is a real good way to end up with a stuffed double.
When gradually working up to a desired velocity, bolt actions can show signs of excess pressure with tight bolt lift etc and usually warn you before damage is done.
Doubles will show signs of excess pressure by springing the action open and if the pressure is high enough, the rifle may already be damaged.
For anyone wanting to boost the power of the 375 flanged mag I'd suggest you do so buy going to a 500/416 or 470 or 500 NE.
Otherwise if you feel tempted to increase velocity/energy/pressure of an existing cartridge, do it with a bolt action not a double.