Is a 7200rpm USB 3.0 drive going to have very similar performance to an internal 7200rpm drive or is it still going to be noticeably slower?
USB 3.0 has an upper limit around 5.0Gbps. SATA III has an upper limit of 6.0Gbps. Regardless of overhead these rates are far higher than what a mechanical HDD can sustain for large transfers.
Most mechanical HDDs won't be able to sustain more than about 1.5Gbps (HDD Speed results). So I doubt you would notice much difference in performance. Real world performance would be affected more by the HDD, chipset and drivers (be sure to keep your drivers up to date).
Just remember to treat your external HDDS gently. Don't knock them while they're running, this could damage the platters. I still tend to eject my usb disks that I use for backups just to be sure they stay reliable.
USB will always be slower than SATA because of protocol overhead, at least. You also must consider that USB is "one transfer at a time", which means any other device connected to USB will degrade performance of the USB-HDD.
While theoretically using 1 USB root for 1 USB hdd might yield good results, in practice every computer has a plethora of other devices connected to USB.
But, even if you buy an external drive (which usually houses the slowest available model), you can always rip open the enclosure and take the hdd out. Unless your computer is a laptop without a 3.5" bay and the external drive is 3.5" (which would explain the price difference). Then an eSATA port would give the best performance.
This answer is getting criticized...
The main support to my answer is my personal experience: in the last 12 years I've used many usb 2.0 / 3.0 external HDD for backup purpose.
In my direct experience external usb drive have always been waaaay slower that internal drive.
I know that when I need to backup 1 o 2 Tb worth of data to an external usb drive (doesn't matter if it's a 3.0 usb) the only way to do it fast, is dismantle the usb drive, and attach the HDD directly to the PC via ata/sata.
This is just my experience, but maybe I'm overlooked something...
this night I'll try to get some numbers to support my claims.
Update: At the moment I don't have external USB 3.0 drive, just old usb 2.0 HDD case, and new eSata HDD case, so I'm unable to produce any useful data to support my claim.
(clearly I'll try to produce some data whenever I find a spare usb 3.0 case)
Original answer:
Sorry but NO, an external USB 3.0 drive can be waaaay slower that an internal drive.
This is especially true if you have many small files.
I know this from my experience, because I use external drive as a backup, and any external USB drive is waaaaaay slower that an internal drive, or an external eSata drive.
To support my claim I've just made a simple test: try copy 10'000 small files to both external and internal storage. (each file is 400 bytes)
For external storage I've used a Sandisk Extreme Plus 128Gb flash card (it write data at 80Mb/sec, faster than many mechanical HDD, and no moving parts...)
For internal storage I've used a 1Tb sata HDD (Samsung HD103UJ).
Copy 10000 files the external SD via USB 3.0 took 150 seconds (66 files/sec - 0.03 Mb/sec).
Copy 10000 files the internal HDD via sata took 3 seconds (3333 files/Sec - 1.30 Mb/sec).
So, using an external HDD connected with Usb 3.0 is ok if you have few big files.
But if you have many small files, of if you plan to use the external drive as a backup, be prepared to wait long time.
(on my boot HDD I have about 484'000 files... copying these on an USB drive at 66 files/Sec would take more than 2 hours if all these file where just 0.5kb each)