Fast discs are always best. Then it is up to how big your solution is.
This I how I setup my hard drive like this
C: OS + FileMaker Server installation
D: Live database
F: Files stored externally in your solution
Can you tell me if your setup is dedicated or virtual setup?
Dedicated or Virtual shouldn't matter. The way you set it up is identical.
Consumer level SSD or iSCSI drives?
That's not a straightforward comparison though. iSCSI is a protocol and the drives that it uses under the hood can be SSD themselves. So you can't really compare a type of drive with a protocol to access it.
Describe a bit more what configurations you are after. You also mention dedicated vs virtual in a later response. What are your choices?
The key here is that FMS benefits from really fast hard drives. But they also have to be extremely stable otherwise you risk corrupting your solution and suffer from bad performance. If you use iSCSI to point to a bank of slow spinning disk across the LAN (or even worse the WAN) and that network connection is brittle then you will be in a world of hurt,
If however you are setting up your server with access to a stable and fast SAN then that is good. Good SANs have traditionally used Fibre Channel to connect to them which offers great speed and stability. iSCSI: not so much, but it was a lot cheaper to use iSCSI.
So consider it carefully and err on the side of stability and speed, even if it costs more in the short term.
I need to know which disk type to use on the fm server. Consumer level SSD or iSCSI drives?
How much load will your server be under? Note that "Consumer Level" hardware is not as thoroughly tested as server grade hardware and that they advertise shorter lifetimes for consumer level drives.If your server has a few users accessing it then go for server level hardware for the peace of mind and longevity.
As far as storage performance goes you should post more information about your hardware or consult a server IT consultant (or shop) as there are a few different parts of your hardware set-up that will effect storage performance. And note that if you are buying the drives yourself then sequential transfer speeds that most advertise don't tell the whole story for drive performance for databases, random IOPS and latency is also an important factor. The sky is the limit on what you want to spend... there's a good table that includes the ridiculously expensive here IOPS - Wikipedia