Short answer: the bluetooth scanner is much better, but it's not perfect.
Longer answer: DL was written for old Macs with external iSight cameras. The built-in cameras on the new Macs are different, and pose some challenges for DL. Wil Shipley claims to have written a workaround
(search in-page for "focal"), but in my experience the built-in iSight just isn't usable for scanning barcodes. Sometimes it scans immediately, sometimes it takes many tries over a 30-45 second period, sometimes it will not recognize a bar code at all. This is my experience, though, and is not in any way a negative opinion of the DL folks. On the contrary: I think it's amazing that they've made it work at all and as well as it does.
The Flic bluetooth scanner is really quite nice: it's comfortable and quick to use. It nearly always scans in less than a second, occasionally (1 book in 50 or so) takes a few tries before succeeding, and very rarely fails to read a barcode. It is indescribably useful for scanning an existing library: no need to haul books over to the Mac. The downside is, mine is flaky. It frequently fails to bluetooth-connect to my iMac. DL gets itself into a mode where the Bluetooth icon cycles colors, as if it's trying to talk to the scanner, but they never actually sync up. This requires a hard reset procedure on the scanner, and resetting the Bluetooth stuff on the Mac, about 10 minutes of work. After that the scanner will work fine for another week or maybe even a month. I'm the only person who has reported this problem, though, so it might just be my scanner or my iMac or even my karma.
Bottom line: if you have more money than time, definitely get the Flic. If you're willing to be patient with the camera, and enter numbers manually when it doesn't scan, DL with iSight is still pretty nice.