They are basically identical in a given year; the difference being more features on the 900s so they are nicer. The 900s continued after the 700s were discontinued, and the cars continued to evolve with things like better oil distribution in the B230L than the B230K engines used when the 700s were still being made. The FAQ has a section on model identification which will give you a lot of the details.

Maintenance costs and reliability should be similar for a given year, with the exception that the extras that are in the 940s aren't there to break in the 740s.