the only close-to-reliable external or internal drive is the one... you have backed up. Or at least mirrored (Raid1).
You can NEVER rely on "statistical reliability" of any mechanical storage. They just break. Without warning. Your data will be gone even if your drive will be the first to go down in this world - so even if anything had 100.00000% reliability before, your data will be lost.
And I'm pretty sure you understand there is nothing with 100.0000% today.
Back up, mate. Storage costs nothing today. Buy identical drives, two of them, have your data stored twice. Don't put all eggs to the same basket. NEVER.
Whilst the theory of the above is true, I think you have gone off in a slightly different direction than the actual question.
Portable hard drives are not used for the primary location of data storage. They are uses either to backup from a primary location to the portable HDD which is then stored somewhere, or they are used to copy data, or more specifically large data from point A to point B. Both scenarios the data already exists at another location. I use my portable hard drive to copy data between my home and work machine, and also to image machines I get called out to fix so I can transfer that image onto my primary machine and then into my backup schedule.
Regarding the drives, I very much doubt there is any difference in the actual internal HDD inside all of the passport series. All of them including the enterprise version can be powered from a single USB2 port. If WD could get a 7,200rpm HDD from a single USB2 port then I would suspect they would standardise on 7,200rpm across the passport range but to my knowledge they are all 5,400rpm.
If you look on Western Digital's site they promote the enterprise drive has having "Windows to go" - a feature which will allow you to run a virtualised environment from the drive on a host PC without leaving a trace on the host PC. Nowhere does it talk about any additional reliability the drive offers over the non-enterprise drives. On WD's SAS drives and RE drives they do talk about an increase in reliability.
Depending on how sensitive your data is you may want to avoid the Elements as it does not support any form of encryption. If you need more than 500GB that rules out the Passport Edge and the Enterprise. I have just bought myself a 500GB Passport as it supports encryption and was well priced. But in terms of reliability there will be nothing in it between them.