As with everything in this world, YMMV or in the classical answer “… it depends…”
As far as the slicer is concerned. All files that are migrated into the slicer end up being a mesh. The difference is that a STL file starts out as a mesh whereas a STEP file has more precise measurements overall.
It may help to understand the distinction between a STEP and an STL. An STL is a mesh made of triangles(sometimes imprecisely referred to as polygons in some slicers) that are linked together. A STEP file on the other hand is a series of mathematical equations mapped out on a 3D X,Y,Z coordinate system. STEP files provide much more detail and far more accurate dimensions when describing things like lines, curves and arcs. STL files have to try to “approximate” a curve by cutting down the shape into finer and finer mesh.
If you’ve ever taken let’s say a vector drawing like an SVG or DWG file and scaled it, you know that it will look the same at any size. However, if you’ve ever tried to upscale a bitmap image like a JPG, GIF or BMP file, you’ll see that the pixels become blocky. Same principle applies to a STEP vs STL. A STEP file is vector-based while STL is comparable to Pixel-based.
There are times when having a low poly count has some artistic benefit(see low-poly prints on Printables). However, the fewer the polygon count the smaller the G-code file and therefore the less work the processor in printer has to do. However, this can be at the cost of resolution.
Here’s some examples of what I mean.
These two objects were done in CAD. The the smooth image left is a STEP file and the triangle one on the right is an STL. As far as the printer is concerned, both parts look close enough.
If we bring in both files into the slicer, they again look close enough unless you zoom in. Here you can see the STL on the right has slight ridges which you can see in the print if you look real close.
The real magic happens when I try to simplify the model.
Under the STL, I only got 232 polygons but the STEP file I got 816. NOTE: These were the same files exported from Onshape so one might see different results from let’s say an application like Blender or MeshCAD or Fusion360.
On a personal note. If I have a choice, I always use STEP files. But full disclosure, 90% of my 3D printing, I am looking for dimensional accuracy to print parts, not artistic items that have lot’s of organic structures. A STEP file gives me much greater control over exactly what screw hole sizes I print as well as lining up two parts that are going to be joined together. That said, I’m being totally OCD on that topic because there is no reason you can’t get away with only using STLs.
And last, file size. This is all depending on the object. If it’s a really tough part with lots of fillets, chamfers and curves, the STL sometimes is smaller. But in the end, my experience is that the G-Code for simple geometries tends to be similar in size between a STL vs a STEP and that makes sense if you think about it for a moment. Once the printer has the G-Code, it’s all a series of XYZ points anyway and the printer can’t tell the difference, it’s just moving the print head to where the code tells it to go.
