This depends, at least partially, on what you will be flying after you complete the training. However, that is not something that should be cast in stone. It is my opinion that the training would best be done in a glass cockpit airplane because everyone who uses airplanes for purposeful transportation will eventually be flying a glass cockpit, so why not train there? It would probably be simpler to do it with steam gauges, if that is what you have been flying, but the glass is a nice challenge and once you learn it there is so much more capability there. Certainly if you learn in one you can transition to the other, but that will take some effort.