The stuff I buy online is usually not stocked by any LFS near me or is a specific item that I can save a lot on. But I regularly hit most LFS within a three hour drive, so I have a huge variety available.
I also don't buy online when I have to pay high shipping fees. I'm lucky that a lot of the online vendors located in Florida will do $15 overnight FedEx, but I still need to buy $100 worth of frags to make it significantly cheaper than the LFS. Plus, there's a frag swap every few months somewhere near me that the same vendors will have a booth at.
The LFS is handy for quick buys. I bought a test kit, a PC bulb and a Hydor Flo from several different LFS in the last few weeks, all of which would have been 10-50% less online. But I could pick each item up, take it home and use it. That convenience was worth the extra cost.
On the other hand, I bought BRS rock instead of that from my LFS, even though the price was similar, because I liked the BRS rock better. So it really depends, I purchase based on price sometimes, convenience others and just plain personal preference some other times.
Jeff