I have flown the 300 and I currently fly the 680.
The 680 is nowhere near the 300. Rwy performance is -as already mentioned - the strong point of the 680, sometimes we are airborne within 500mtrs run.
Cockpit: The Epic in the Sovereign is a pain. (yes we have 4.2, still I hate it)
The proline 21 is the best avionic suite I´ve seen yet. The complete layout of the 300 cockpit is simply great, and the Proline is used to its capability.
The 680 could have been good, but Cessna decided to build another oldschool style Cessna. (Eg. Cabin pressurization - the 650 controller with loads of dials and knobs and switches, you manually have to set either cruise alt or det alt. the LH Pilot cant really see diff press or cabin alt since they are in the rh corner - on the 300 it takes just a flightplan in the FMS and youre good to go, diff press and cabin alt are displayed in the MFD)
Checklist: the best I had yet is the 300 stuff. With clear flows, crossreferences, modern layout, you name it. The 680 checklists are a pain, about the same than in a C500 from 1970.
Cabin: the 680 is no match. The 300 wide cabin is very close to the 604. Flat floor, good seats etcetc. Only thing that is better on the 680 is the seal on the door. No factor in the 300 if you have the door between galley and cabin. Interior quality of the 300 is a class better IMO.
Engine starting: the 680 uses electric starters that give a lot of problems. They are simply undersized. The pneumatic starter on the 300 is a proven thing. Start time of one engine in the 680 is about what you need for both in the 300.
Build quality: 680 is a Cessna - need I say more?
Range: forget about the 2900 nm Cessna says. 2400 or 2500 if youre brave. I have done 3320 NAM in the 300 and landed with 2000 pounds remaining.
Refueling: electronic fuel panel in the 300: dial in what amount you want as total load, the airplane does the rest. 680 basic system.
Brakes: very good on both airplanes.
Baggage space: more an the 300 but loading is easier in the 680. But you have access to it in the 300 during flight.
All in all, they are both marketed in the same segment, which I find to be strange. The SOV is simply a bigger XL, way smaller than the 300. (they are alsmost identical in lenght and span, but cabinwise...)
And the 300 is pure joy to fly, sort of a sports car. The 680 is like a big truck compared.