Which Neu for my CRT?

[riceman]

I've searched through the threads hear and it seems the more I look the more questions I come up with. It's obviously been done before so forgive me for posting again for my own benefit. My Neu 1515 1Y is running great in my LST2 but now I'm building a CRT conversion and wondering which configuration I should go with:

1512 2d (2600kv) on 3S

or

1512 2.5d (2000kv) on 4S

This will be used specifically for racing. I have an extra Mamba Max that I'll be using for my controller if that makes any difference in the selection. Any pros/cons you can input will be appreciated. Will I see better runtimes on the 2.5d? Enough to compensate for extra weight of the additional cells?