The external/desktop model has a 1-year warranty, slower than the internal, and can be easily plugged in to any USB port.
The internal drive has a 2-year warranty, has excellent performance (reads and writes over 150 MBytes/sec), but must be installed either inside a desktop computer or plugged in to a drive docking station.
The Western Digital "Black" 2TB is $110 today.
Their "Black" drives are some of the best consumer-grade drives they make, and includes a 5-year warranty.
This latest model (WD2003FZEX) normally sells for $120 to $130.