jaap,
I believe the answer to your question will specifically lie in your own personal work ethic, your learning desires, and the curriculum of the program.
Some programs may give their students more mileage out of a one year program, whereas other programs might be more beneficial to the student if done for two years.
Traditionally, the one year program is an exceptional program and a good learning tool for students who wish to convert their academic dentistry to actual clinical settings.
Some GPR programs only give the option of an additional year to select students. For example, some GPR programs may accept only 15 students, and then have positions for 2 or 3 students to continue on for another year.