Apple, hands down. Though frankly it seems that soon stock will be fairly easy to come by, as compared to just a couple of weeks ago. I got mine at an AT&T store without an order, but I got lucky. The 16GB is a little harder to get than the 32 GB still, but it really does seem like supply is finally catching up with demand.
I ordered two non-premier 32GB iPhone 4s through AT&T and it only took three business days for them to arrive. It definitely beat the hassle of trying to find a local store that had them in stock.
I concur. Order from AT&T, it's so much faster than any other option. I got mine in 6 business days (received Aug. 8th) when people who ordered from apple same day and waited more than 2 weeks at minimum.