Lets not forget the all important factor, so important to B6 management, which is the establishment of a lower payscale for the pilots. They saw this as an opportunity to combat the "creep" in crew costs as pilots wages inch up with seniority. It also gives them the future option of moving flying from the 320 back to the B-scale 190's.
The 318 would have paid the same as the 320 since it's the same airplane. It would have been a hard sell using the "but it's only an RJ" argument with the 318.
You are absolutely correct, it had nothing to do with aircraft economics, intended route structure, aquisition cost etc, etc. It was all about the payscale.