AOA. It is not fair comparing Mark X with the other two cars here, it is in a diff. league altogether. The driving dynamics and everything else of X is just superior than the other two. If you plan on getting one make sure you get a clean car and one which is in tip top condition mechanically too otherwise i can be a pain in the butt. We bought a black 250G grade Mark X last year for a couple of months as me n my brother were in karachi for some time. Bought it from my elder brother's friend who lives just across the street.
The most prominent difference b/w the two variants is obviously the engine capacity where the 300G is 3ltr and 250G having the less potent 2.5ltr engine which is sufficient enough for the car, i'm saying this as my car used to lose traction at the rear tires every now and then, which is what i loved the most about this car. 300G grade comes with i guess standard smart card entry system and also some cars also have sun/moon roof. I have seen few 300Gs with Auto park assist too and also they come with i think standard 18'' wheels.
The current market for Mark Xs is not what it used to be. I got mine for i believe 24 and sold it for 23 something a couple of months later. Nowadays i think u can find a clean one for somewhere b/w 21-25. I hope this bit of info help you in some way making up your mind.