OK, the 1515 is .3" longer than the 1512. The 1515 is also heavier, by a bit. Enough to make a difference anyway. The 1515 also makes more torque. From my experience, the 1512 is plenty of motor, but if your project weighs over 10lbs or so, I'd probably take a closer look at the 1515. I just got the 1512 for my truggy, but it's an 8ight and it's pretty light at just under 10lbs in race form with the battery.

The Wye wind is going to be a bit more efficient, but negligibly so. Genearally speaking, a delta wound motor will put out 1.7 times the number of rpms per volt when compared to a like Wye wound motor. Conversely, the Wye wind will put out more torque. When it comes down to it, just choose the one that hits the kv rating that you want. They're all good. I went with the 1.5Y (1900kv) because I am replacing a 9XL and want 8XL performance. I could just as easily have gone with the 2.5d (2000kv), but it is a race setup and I'm taking every advantage that I can get.

I also went with the smaller 1512 because it is lighter, draws less current than a 1515 of similar kv, and it has plenty of power for my application. If weight and runtime, not to mention price, are not a consideration, I'd go for either the 1515 1Y (2200kv) or 2.5d (1700kv), depending on the battery configuration that you want to run primarily. The 1Y for 4S and the 2.5d for 4S or 5S lipo. Many have run the 1Y on 5S, but I think it's too much.