The Astra will definitely be easier to own (and this is coming from a Ford guy... Volvo=Ford, Opel=GM)... and it comes with a smaller range of engines, too, which should be more economical.

Just look out for the usual stuff, suspension, bearings, brakes, engine roughness, aircon problems, cooling problems in traffic, etcetera. For better peace of mind, you may want to consider buying a manual transmission model, to avoid servicing headaches which are common with all secondhand automatics (regardless of brand).