So, your question is
How much advantage have you gained by using the longer runway.
And then you have said to ignore cost.
So I'm not actually sure what "advantage" you are referring to. If you are referring to performance advantage in the case of an engine failure, the answer is what advantage? Either runway is safe in the case of an engine failure. One will have more thrust, so will be harder to control, but will climb better. The other will be easier to control but not climb as well, but will still clear terrain.
But, are you asking which runway is
safer?
I would say the long one. For various reasons, as follows:
1. An engine is
less likely to fail at lower thrust. I know many pilots feel better with more thrust, but all they are doing is increasing the likelihood of an engine failure. An engine at lower thrust will also suffer less damage from bird or foreign object ingestion.
2. On the longer runway ASDA for an RTO is less likely to be the limiting factor, so less braking may be required reducing the possibility of a brake/wheel fire or tyre deflation. Again, safety improvement.
3. What may conspire to kill you on the day may not actually be the certified standards of engine failure/stop/go scenarios. In fact, none of these should kill you because the aircraft is certified not to. What may kill you is a multiple failure, an error (eg thrust calculation error), a sticking brake, tyre failure, manipulation or handling error, or other event that is not possible to plan for. In that case, having more runway to sort it out may save you.