The 5700kv on 3s will have more power by virtue of the higher voltage. More power means it will pull higher gearing. Have you done any gearing calculations? I ended up with a 1/12 pan car and even with the highest gearing that would fit it took insane rpm to make it go fast. If speed runs are your goal you may end up with the 7700kv on 3s to get the rpm needed. There are other motors available with kv in the 9000 range as well.

Btw the pan car I had was a real handful at high speed. Getting there wasn't easy either, the power just made it want to spin out. I bought out someone's collection and just used parts that came with it. I believe it was a 9 turn brushed motor that I ran on 7s NiMh and geared to the max it was still only around 45mph bouncing all over my fairly smooth street.