Question - "'Which is better: failure 10 kts before V1 and continue T.O or failure 10 kts after V1 and abort T.O?"
Answer - NEITHER
Assuming that you were in a Balanced Field situation where Accelerate-Stop and Obstacle limits are equal, reject from V1+10 would inevitabely result in an overrun. For a dry field you will have the small bonus of reverse thrust being available, but this would probably be insufficient to dissipate the additional 10 knots of kinetic energy.
For the continued Takeoff from engine failure at V2-10 you MIGHT get away with it, as the margins for the continued takeoff are somewhat better, BUT, what if the original V1 was Vmcg limited and you're now attempting a continued takeoff with inadequate directional control? - You crash. And again, even if not Vmcg limited but field length limited, do you have the performance available to accelerate an extra 10 knots with an engine inoperative from Vef to V2, I doubt it - You crash.
And what of the comment somewhere here that we regularly reduce V1 for Wet runways? True, we do, and apply a usually hefty weight limit to go with it, but, on a dry runway your limiting weights were calculated for the higher V1 - You crash.
And what of the enquiry afterwards (assuming that you survive)? - You crash.
The numbers work, they're well proven, use them properly, know the means by which your aircraft was certified, and don't fiddle with them.
Regards,
Old Smokey