a) Is there a big difference between consumer HDDs and enterprise ones?
Yes there is a somewhat slight difference, as Enterprise Drives sometimes are more expensive than Consumer Drives, but they're built with the intent of the Server always being online as with Consumer Drives are built with the intent that the System is powered on and off quite frequently.
b) Also, is there a difference between sata and sas?
There is a difference as they are different connection types, if you're asking about the Drives themselves as long as they are traditional platter drives no they're not too different and function in the same way as they have for years. The difference is in the speed based on the peripheral connection SATA/SAS.
c) Are there any statistics regarding brand reliability? Is WD more reliable than Seagate or HGST?
This question runs more into debate than a hard proofed answer, in general most HDDs are reliable to a certain extent. Yes sometimes you do get a bad batch of Drives of which can cause backlash, but generally as I said it's up to debate which are better.
My Dedicated Servers run off of Western Digital drives, I've no problem with them. My Desktop runs off an assortment of WD, Seagate, and HGST Drives as I primarily got what was on sale at the time.
The reliability factor isn't really the issue as all hard drives will fail or die after a certain amount of use. It's inevitable as nothing lasts forever, it's what you do to prevent them from dying right away is the key.
Another thing to mention here that Largo didn't cover is the difference in Warranty.
If you're placing a server into production that may function for 2-4 years, would you want a 1 year warranty or a 3-5 year warranty.
Most consumer drives may only come with that 1 year warranty, where-as the Enterprise drives may contain a 3 to 5 year warranty which significantly reduces the cost of replacing a drive if it is part of that "5%" if you will.
Consumer and enterprise drives can differ in performance and error tolerance. Manufacturers take into account how much (ECC) correction is needed as this can have an impact on read speeds, processing power and physical storage space possible on the drive. I agree on the warranty info, and some consumer drive sellers offer extended warranties, but no more than 2 years I think and sometimes the manufacturer provides this already. Getting a new drive under warranty takes forever… There always needs to be spare drives for that...