I suspect they are about equal in quality but the built in one may be easier to use.
Neither is the ultimate in quality.
For that you need a dedicate film scanner.
The big players of 10 years ago -- Nikon, Canon, Minolta, Acer, seem to have largely pulled out of the market. High secondhand prices for USB versions reflect that.
However if you have a desktop computer and can fit a SCSI card, there are some quality bargains to be had.
The only new players seem to be Plustek whose USB models currently offer the best combination of price, quality and connectivity.