MCC, you'd written "more confused than ever" and my "dupe" post was adding only MORE confusing issues to consider, so I wiped it out. The External Drive Issue started this - I should stick with that.
Like Dan said, "Get what you can afford" but I'd also have you look at the BARGAIN BASEMENT sub-forum and notice how often and how varied prices can be.
We have seen a LOAD of external 3Tb drives closer to $120 than more than $150, and those prices go up and down every few weeks. If there is no immediate need, then I'd counsel "wait".
But I'd also counsel "USB 3 Only" - their prices should NOT be that much more. Look at that HornetTek External dual-drive case, for example.
The USB 2.0 version is $30. The USB 3 is $50. Sure, that seems like "almost double the price" but in reality, "it's only $20." And we're seeing that $20 difference is fairly common - and even then it's probably price-gouging because the electronics and plugs cannot be but pennies in difference.
MY EXTERNAL CASES...
I use external cases because I have spare hard-drives. Old ones, dust collectors. My choice for external cases is based on (1) commonality of the Electrical Plug (so I don't have to lug power-bricks, etc) and (2) price.
I have a few 'docks' or 'sleds' and those are very handy BUT I find them to be flakier with shorter lifespans.
One of the problems is that, in a 'dock' like the well-regarded Thermaltake Duet, I can't 'see' the connection being made. If I'm off a bit, I may not realize it until I've damaged the connectors.
Which connectors? Well, screwing up the dock's isn't a huge deal. Fine - I'll waste money.
But what if I break off the drive's connectors? Uh. Sometimes, NO MONEY replaces that drive's contents, and suddenly, that's what Me & My Dock have done!
So, I much prefer external enclosures where I can 'see' the connection being made, I can carefully align those and hopefully have a longer-lasting product.
If you don't swap drives out of a dock very often, then this isn't a concern. Of course, it only takes one broken connector off of the drive and suddenly it is The Only Important Drive you might have! Argh...
External enclosures, by the way, don't need to be sealed up. I can leave them 'open face' easily. Of course, I can't sling them around by their USB cable, but I seldom do that anyway.