The differences between two-year and one-year MBA programmes reflect the traditional time taken for a master's degree in various regions (two years in North America, one year in Europe).
Generally speaking, schools with a one-year programme (and note that "one year" can mean anything from 10 to 16 months) argue that there is little difference in terms of areas covered and time spent in class compared with a two-year programme. Certainly while the first year of a two-year programme is a very intensive experience, the second - which is is given over to elective courses and job hunting - is much less so.
Overall, given schools with similar reputations, there is not that much to choose in terms of what you learn and the quality of the experience. The main advantage of a two-year programme is the internship (job experience within a company) between the two years. This is particularly important if you are using an MBA to change career. An internship, which is often a vital first step to a full-time job, allows you to gain experience in the area you want to move to. As the questioner points out, if you are not planning a significant career chance and have considerable work experience a one-year programme might be more appropriate.
In the end it all comes down to personal circumstances and choice. A one-year programme is ideal for a quick career boost but a two-year programme allows better opportunities for career change and is also reckoned by many students to be a more worthwhile educational and personal development experience since it allows more time for reflection.