Depends on your CPU...If you have a gig of RAM and fast processor - you can go to level 6 - no problem...
And as the someone else pointed out the returns arent as great in the upper levels - its not a linear thing...you might only pick up a few extra MBs in the 6-9 range...
If you have a older slow pc - you may not want to wait...
but to answer your question - no difference in quality...