How many times do you revise your manuscript? How many readers do you have go through it? At what point do you decide it's ready and move on to querying?
If you decide to self-publish, do you put as much work into it? Do you still have it critiqued by multiple people who know what they're doing? I think it's way more important to do those things when choosing this route.
Have you ever ran across a self-published book and wished you could help re-write it?
That is what critique groups help prevent.
If you aren't a member of one-find one to join or start your own. You not only get wonderful suggestions for perfecting your manuscript, but get to enjoy reading others. (Even if they sometimes make you feel inferior because they're so amazing. ;-) )
Workshops and seminars are wonderful, but joining a critique group was the best thing I've done for my writing. My critique friends are awesome!