Depends on the application and intended prop choices. Generally speaking, a low KV motor will have a huge amount of torque compared to a high KV motor, it isn't about which one is faster, a lot of it comes down to how you use the power. From what I have seen, you can get a stock supervee motor doing 40mph on 12 cells, you may get a higher KV motor to do the same but you could also be using a lot more power to do it if propped too high. High KV motors are usually reserved for things like riggers where you can run a tiny prop but have it spinning at a great rate of RPM, just because it wants to spin at 30-40,000 rpm doesn't mean it will if your prop is too big which results in massive amp draw and a very short runtime. Personally I would still go the ammo, it is rated to 2300kv whereas the 6xl is nearly 2800, I think the ammo is a safer bet.