Again - it can be down to more subtle things - they may have the same "standard" credentials (such as ACEA A4), but the manufacturer specific approvals may be very different (and this is where a lot of cost and effort is involved in producing a fluid to meet the more stringent OEM standards) - look for the MB (e.g. 229.3/229.5) and VAG (e.g. 503.00/507.00) credentials. These are generally the more tough manufacturer specifications and cost a lot in engine testing and also need much better components to pass. They have specific tests covering different aspects of performance from fuel economy to sludge resistance, which all have their own pass/fail criteria on the various parameters, from engine cleanliness, wear or fuel consumption - it's a pretty complex business!
General guidance would be the more credentials, and the higher the number, the better (such as MB 229.5>229.3)! Be careful also to look for one which is approved against the specifications, rather than suitable for use in... Suitable for use may mean it hasn't necessarily been fully tested, and it certainly doesn't have a formal approval against the specification. Often the cheaper oils say suitable for use, while the top tier versions are fully tested and approved...
Hope this helps!