Well Synology has just been awesome with compatibility and updates, but hitting the first signs of obsolescence. It looks like Synology systems older than June 2014 have an 8TB hard disk limit. I know, I know, that seems like lots of capacity, but it's not well documented if you look at the Synology compatibility lists you can see this note. So beware as you buy drives.
And as always have a backup plan and run with some sort of hard drive backup. For us what we are doing is putting 10TB drives into the DS-2413+ and then migrating the smaller disks (6TB and so on) down to the older models.
I also can't find compatibility data for Drobo but they've been handling up to 8TB well.
In terms of what drives to get here's a reminder:

1. Use a reputable vendor, when you buy from Amazon make sure you are actually getting it from them. Just bought a set and they came in hand bubble wrap as they were probably broken up from an OEM kit.
2. Get a drive with a five year warranty, it may cost more, but you aren't Backblaze, do you really really want to deal with rebuilding your array.
3. Look for high ratings particularly at Newegg, the Amazon reviews are pretty useless because they combine lots of product categories and of course there are many spammers there.

Given that here are some drives to look at:

1. Seagate Enterprise Capacity 512e. At $410 it's a relative steal, with prices around$40/TB being good. This is really a 4K driver underneath but emulations 512 byte sectors, amazingly it's right now cheaper than the Iron Wolf but has 2.5M hours MTBF and a five year warranty.
2. Seagate Iron Wolf Pro. This is a NAS drive at $420 at Newegge also 7200 but has the 5 year warranty and 1.2M MTBF 3. Seagate Iron Wolf. Wow these are cheap at$367 at Newegg and they are faster 7200 rpm, but only have a 3 year warranty and 1M hour MTBF, but are designed for 24/7 use.