Neither.
Do not try to tap sheet metal. It's not thick enough to create a good grip on the screw.
There are other ways of creating a tapped hole in sheet metal components, but you haven't given us enough information to make any suggestions.
You're going to need to provide more details than that. Are we talking about an attempt to actually thread a simple hole in a piece of sheet metal (I would suspect that would be dependent on the thickness of the sheet metal and the size of the fastener)? Now if you're talking about punching a so-called 'extruded hole'...
...that could provide more 'depth' to thread, you might be able to use either fine or coarse threads, again it would be dependent on the thickness of the material and the diameter of the fastener.
Questions:
1. What is the application?
2. What type of material?
3. What is the gage of sheet metal?
These questions will help guide you in selecting the type of fastener. As mentioned above, there are screws specifically for sheet metal and there are process options for the metal itself. These include drawn holes, rivnuts, PEM nuts etc.
You are not going to get adequate threads in that.
It would sure help if you could get those of us who lack mind-reading capability, at least a little more than a hint of what you are trying to do, because perhaps this might rule out some of the many options that you have.
Rivnuts.
Weldnuts. (Used in automotive all the time. Commonly secured using projection-welding, resistance-welding, "spot-welding".)
Through-hole both parts and use screws and nuts (and washers), and forget trying to thread the metal.
Through-hole and J nuts. (Google it.) Works if the attachment point is close to the edge of the plastic part and the thickness of the part is in an acceptable range. Used for securing motorcycle fairings to underlying metal frames all the time.
Through-hole the sheet metal and thread the plastic piece, using self-tapping fasteners similar to wood screws, or possibly actual wood screws, depending upon the consistency of the plastic that you are trying to attach. Requires the screw to go in from the other side.
There are lots of ways to do this. More information = more helpful answers.
"Anyone have difficulty trying to thread through 304SS with 410SS screws?"
Did you not hear us? Do not try to create ANY thread in sheet metal. It will not hold. Period.
You have three options:
(1) Go through a clearance hole in the sheet metal to a nut on the other side or,
(2) Add a "thread insert" to the sheet metal or,
(3) Weld a nut on the far side.
Those are your options. Fine or coarse thread does not matter.
It all depend what the sheet metal is used for. I have seen seen and also used successfully screws of sheet metal for panel doors and air ducts, although, I always had a preference for rivets. It all depend on the application of the sheet metal.
Tapping sheet metal is bad practice.
Since we have no clue about the application. Suggest match drilling and riveting.
The other suggesting using floating nut riveted.
