I'm rebuilding my current system- I have a 4x burner now and upgrading is a viable option.
I could purchase either 2 NEC 3550a's or 1 Sony DWQ120A / NEC 3550a. I typically read/write DVDs and write backups of photos- 2x each. Therefore I'd want to use the 'multiple burners' option in Nero.
Since some DVDs read well in different drives I would lean towards the DWQ120A/3550A combo.
However for compatibility with double burning DVDs I'd be tempted to go with the 3550A combos. Plus- there's firmware that removes the 2x lock on ripping.