E-SATA is the fastest and USB is the most versatile. With Vista you can Hot swap an external SATA (E-SATA) HDD. USB is just about 100% universal. Meaning you can take your HDD with you and axcess the data on the HDD from almost any computer.
My choice is An external HDD that has E-SATA and USB.
Rosewill RX-358-S SLV (Silver) 3.5" SATA to USB & eSATA Ext. Enclosure w/Int.80mm fan $24.99http://www.newegg.com/Product/Product.aspx?Item=N82E16817173043
Western Digital Caviar SE16 WD6400AAKS 640GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive $69.99 http://www.newegg.com/Product/Product.aspx?Item=N82E16822136218
You might want more or less storage.
The enclosure comes with all the cables you will need and a power supply.
Another good choice would be a dock. If you need to transfer data to multipal HDD'shttp://www.newegg.com/Product/Product.aspx?Item=N82E16812264002
The enclosure is ON SALE RIGHT NOW. Regular price $49.99