Remember, there’s lies, damned lies & statistics. Vendors will play with numbers to make their position look better. For the most part, signature-based AV is all about the same. The best solution is a mix of signatures, heuristics and behavior-based anomaly detection. An additional feature that is useful is whitelisting.
When it comes to competition, every antivirus software company has their own genius programmers who always think of how they could improve their products by creating new feature such as reducing its consumption to the processor.