I prefer on-line, I like the guarantee. Also there is a better selection. The biggest plus...The fish are not shipped, then dumped into a tank for me to see and purchase, stressing it out more by netting and bagging it yet again to take it home to me. I have a book (with pictures, all the same species look the same if they are healthy), and there are lots of articles on the web to tell me how to care for any critter.
I do believe in supporting our LFSs... I do check to see what they have, and if they have something I like, I will buy it. Mostly I purchase stuff like food at the LFS, I would be lost without them and I want them to stay open for business.