They will both be the same speed. If you're talking about Serial ATA adapters, then the only difference is the connector type. All data moves equally fast.
external serial ATA is six times faster
External serial ATA known as eSATA is up to six times faster than USB or Firewire.
External serial ATA (eSATA) hard drives is up to six times faster than USB or FireWire.
External SATA (eSATA) is up to six times faster than USB or Fire Wire
50-85 Mbps for eSATA vs 25 - 45 Mbps firewire 400 not sure what 800 goes to but eSATA will still be faster it's just like an internal drive. eSATA is up to 6 times faster than external firewire according to the fabulous Guide to managing and maintaining your PC by Jean Andrews
The highest rated cable is the Dynex 16" USB PDA/Serial Adapter Cable
If you need to use an older serial device, than a USB to serial adapter is required. This adapter has all of the required drivers programmed into it to ensure that the use of your serial device is seamless.
In computers, the bus is the subsystem that transfers data between internal parts of the computer, or from internal parts of the computer to external parts, or between two computers. External bus can be parallel (ATA (and all of its derivations), IEEE-488, SCSI) or serial (USB, FireWire, etc.).
There are many places where one can purchase a cheap serial adapter. One place where one can purchase a cheap serial adapter is at Best Buy or at Amazon.
eSATA (external Serial Advanced Technology Attachment) is a high-speed interface for connecting external storage devices to a computer, offering faster data transfer rates than USB or FireWire. It uses the same protocol as internal SATA connections, allowing for plug-and-play functionality. eSATA supports hot-swapping of devices, providing convenience for users who frequently connect and disconnect external drives.
Connecting an adapter USB to a serial port is very simple. All you need to do is purchase a USB Serial Adapter. These range in price from $6.00 used to upwards of $30.00 new. http://www.Amazon.com provides several USB serial adapters to purchase online.
esata is faster, running upto 3GB/s where as FireWire400 (G1) can only run speeds of theoretical 400Mb/s and FireWire800 (G2) theoretical speeds of 800Mb/s