The Xbox 360 Slim uses SATA II which has an uncoded transfer rate of 2.4 Gbit/s (300 MB/s).
USB 2.0 has an effective High Speed signaling rate throughput of 35 MB/s.
The Xbox 360 Slim comes with a Hitachi HTS545025B9SA00 1.5Gbps (That's SATA I with an uncoded transfer rate of 1.2Gbps or 150 MB/s which is half of SATA II). SATA II is backward compatible but you'll be running at the lower SATA I speed. Moreover the included drive is a 5400 rpm HDD, not an SSD.
Even with this, I expect you would see performance gains from using the SATA drive.
Ideally you don't want to be using an external drive for gaming through USB as this takes more processing of the data going going back and forth, external drives aren't built for constant read/write speed the way internal ones are, and USB is just slower than SATA all day long.
Conclusion: Use the current internal SATA that you have and if you want even faster performance consider investing in a SATA II SSD where you'll really see your storage bottleneck disappear.