I'm not an engineer nor do I have any practical experience with Inconel, but...
Inconel 625 was designed more for its corrosion resistance and 751 was designed specifically for exhaust valves. If you used 625 for exhaust valves, they might survive heat and corrosion, but could eventually suffer thermal fatigue cracks, valve face recession, or creep deformation sooner than a 751 valve.
MatWeb has datasheets for each...
Special Metals INCONEL® Alloy 625
Special Metals INCONEL® Alloy 751