There aren't that many modern AC-powered locos. If you give an example we can be more specific. However, chopped wave will almost always get you better slow-speed performance, more smoke output, and brighter lights.
The downsides are more noise (you may hear a "growl" or a buzz that's absent on pure sine wave.) Also, increased motor heating, and some factory electronics (such as early MTH) will balk or refuse to run on chopped wave power. The best power supplies use a chopped waveform at lower voltages and smooth out to a pure sine wave as you increase to full throttle.
Pittman's are DC motors. So the power coming from the transformer has to go through a rectifier and probably an electronic e-unit before it gets to them. Whether or not you see a difference with chopped wave depends on how much filtering goes on between the power supply and the motor.
Personally I would pick the ZW-L over either of the MTH transformers that you mentioned. The ZW-L is chopped wave, and IMO it runs postwar-style AC motored trains better than the original ZW!
