Most will fit the bill. Depends on how big you need, speed, connection type, etc.. Heck, a hard drive ipod will do if speed isn't important. OWC has some nice external drives. I have one for my 24 inch iMac and works flawlessly. Do you need this for backup? or just to transfer files? If backup, would suggest a 750 or 1TB drive partitioned, half for each system.
Randy