RÜRUP is well possible for US-nationals living in Germany because during duration you can't remove capital out of it...which is one of the requirements accoding to the German-US-agreement. RIESTER is a different matter entirely, because there you can - albeit with penalties - retrieve your capital during duration/before pension-age. The jury is still out if this is sufficient and US-De agreements to make it free of tax-reporting requirements in the US.
Therefore the priority ranking of recommendation for US-Expats in Germany is
- bAV/company pension (provided you are an employee
- RÜRUP pension for all others.