13" & 15" are pretty close. But the 13" is closer to the size of an opponent's thigh, and the 15" more like a body. The 13" will be a bit more elusive, and require more precise shots, the 15" not as much, and will be more solid.
Personally, I like the bigger and heavier, so it trains you to hit harder and helps builds your balance and powerful penetration, but both are great, and both have their strengths. YMMV .