Two ways to look at it.. Sonar 2XL comes with the Sonic Timeworks EQ and CompressorX plug-in's which aren't available in 3. 3 comes with VSampler and the producer version comes with Ultrafunk Sonitus:fx Suite, Lexicon Pro Patheon Reverb plus, per channel EQ, if that takes your fancy. The Bus Architechture is much better in 3 vs 2. The other thing is, I don't know if you can buy version 2 anymore?
Couple of thoughts,
Porter