Any of those 3 will give you an ISCSI target and you can use the inbox Windows Server Backup dor your 2 servers. I think. I am not actually sure about Server 2008 and ISCI but know 2008R2 will backup to ISCSI.
The first 2 support smartware if that is what you want. A simple copy of your documents. If you want a full image backup that allows you to doing a bare metal restore or files or folders, the DX4000 will provide that for the windows machines.
I am not a mac expert but from what I understand all 3 can be seen as targets for time machine though I am not sure if the latest mac version is supported or not.
While the DX4000 is a perfect match for your needs, it is a very old product, very hard to work with if it breaks, and almost imposible to get drives for. If you get it, I would suggest you get the guardian/warranty service for sure.
So you could go with one of the first 2. Use Smartware for the windows boxes. ISCSI target for the Servers.
The DX 4000 is the only one of the 3 does does a full image backup of the clients. If you are using the others with Smartware all it does is copy whatever files you select to the unit.
So lets say you get hit with cryptolocker with the DX 4000 you can restore your entire computer OS, Applications and files before the virus hit. Cause you just never know if you have removed all traces of the infection.
The backups are in a database on the server that crypto can’t touch. Smartware backups are to a network share which Crypto targets first.
WD has some newer products that do full image backups, the DS5100 and DS6100 but they are kinda pricey
Well… I guess the EX and DL NAS’s do iSCSI and I’ve sucessfulyl set-up iSCSI between Windows 10 and the DL4100 without much fuss. Actually, it seemed too easy to set-up.