The Seagate internal is a SATA III drive (6Gb/sec) so if your motherboard also supports SATA III the drive will be way faster than an external one on USB 2.0
Also less clutter on your desk if you get an internal one, and generally more reliable owing to the fact that external ones employ a bridge-chip inside the enclosure to convert SATA to USB, and this can sometimes fail.
Having said that, I've got three WD externals and they've been rock solid for 10 years. I need externals for off-site backups & for easy transfer of docs & pics to other devices.