bmw 3 series vs audi a4

Maintenance costs are similar for both. 3 Series would be a fresher design as that model was launched in 2005 whereas that A4 while receiving a major revamp in 2005 was really the same model since 2001.

BMW 3 series

Depends what you want from a car. The 3 Series is more a driver's car being rear wheel drive than the A4 unless you go for an A4 Quattro. A4 interior would probably give you a better quality feeling than the 3 Series.

Presuming your looking at diesels. 1.9 TDi in the A4 is reliable but very old and unrefined for a premium branded car. 2.0 TDi is still unrefined and suffer from injector problems though there was a recall on these. BMW diesel is more refined than the VAG diesel but can have issues if owners don't adhere to the preventative measures. Turbo failure and swirl flap issues being the most common problems.