I have a bunch of new 6tb HDDs and want to format them as ext4.
I tested formatting a 250gb ssd with gparted and Disks, and the result was that Diska gave me about 270bytes more usable space than gparted did.
However I hesitate to use disks because it always gives a time out error.
If the difference is only a miniscule 270 bytes, use Gparted. As it happens I always use Gparted because, like you, I find Disks keeps throwing errors. For external USB disks I always use Mintstick
The difference in bytes are possibly becase one being MBR and the other GPT or maybe because the default way of formatting they use may based on different tweeking options. You can do it thru terminal using parted or use mkfs but read about tweaking stuff. I haven’t done that my own yet, so I can’t help.
Hi, You’ll notice a capacity difference between Disks and GParted because Disks (by default) uses MB and GB, while GParted uses MiB and GiB.
With GParted you’ll see the same capacity as seen on Windows. While Disks seems to provide more space, difference is only due to the way space is calculated.
Personally I prefer GParted, have become accustom to a gigabyte = 1024. 
I don't think that's the 270 bytes difference. 270B is very few for the difference between GIB and GB. Just do some calcs:
1024B * 250 = 256000GIB
1000B * 250 = 250000GB
256000 - 250000 = 6000 Bytes
Even taking an smaller size of the drive (as it is probably the case):
1024 * 230 = 235520GiB
1000 * 230 = 230000GB
235520 - 230000 = 5520 Byes
Not even a 12GB drive:
1024 * 12 = 12288
1000 * 12 = 12000
12288 - 12000 = 288 Bytes
It you want to maximise usable disk space use XFS
Very true, maybe a small test partition was tried and OP noticed difference in remaining space?
For those who may be interested, Michael Larabel (phoronix.com) performed some XFS benchmarks.
Larabel never focuses on what is important in a file system, stability and tools. XFS is superior to Ext4 in those regards and meets it toe to toes in the performance stakes these days.
It has been my default file system, server and workstation, for 15 years or more.
XFS has a lot of tools and is stable but from a point of view of file preservation ext4 has some advantage, having this advantage makes it have the disadvantage in speed downgrade.
The format of the file-system always depends on what the user needs (and that is why it makes controversial the use of ext4 or xfs as they are different on purposes terms). I prefer having separate partitions and drives for different purposes and each with its own format.
Also is nice mentioning there are other file-systems like ZFS, btrfs, XFS, JFS and worth mentioning tmpfs and FUSE and also it may be need to mention fat32 and NTFS.
Every file system has pros and cons e.g. encryption support, snapshot, etc. it relies on the user needs.