Looking for very light batteries for my buggy.

[BrianG]

Either of those could work providing you keep currents on the lower side. The 3600 pack is good for 108A, but runtime would be rather short. The 4000 pack is "only" good for 80A, but you'd get a tad more runtime. Of course, I am assuming both those C ratings are real-world values unlike some cells that tend to be overworked when pushed to their max spec.

As a comparison, I get around 15 minutes of runtime on my heavy-ish Hyper8 on 5000mAh with a Neu 2d (2050kv) geared for around 35mph. This equates to around 300 watts of average power. To keep the same power level, running a 3600mAh pack would result in runtime of around 11 minutes. You might do a little better since weight is less. And more gradual throttle control helps too.