I have aluminum steps on my Gulf 32 and I can testify to how valuable they can be. The arguments for and against are all sound, and one can be convinced either way depending on their circumstances, but for my boat, and for me, mast steps have been absolutely critical.
To answer your question, my mast steps proceed up the mast as you expect with one on either side. The bottom most step is actually one of those aluminum fold out when you want it steps, but the rest are full enclosure. At the top, there are two steps placed at equal height, and at the height you need to stand in them and comfortably work at the top of your mast. I'm 6'2" and when in them I would the top of the mast is at about my belly button.
The benefit of this is many-fold. For one, I replaced all my standing rigging myself (which is easy and cheaper) and solo. This requires mast steps for the countless up and down trips to deal with connections.
For another, I put a solent stay on my boat and this required lengthy and delicate time at the top of the mast. I must have gone up and down the mast 20 times a day for several days for this project. You can see a photo with me in my top steps at the top of the mast in this blog post about my solent stay.
Stories of Aeolus- Our Gulf 32 Pilothouse: Solent Stay Installation
So, in addition to the benefits of emergency use, I find them invaluable for doing the routine maintenance that too often gets avoided.
Of course, my Gulf is no racing boat though she isn't slow, and the weight of those steps is utterly insignificant to the 6,000 lbs of lead in her keel!!
I also don't know about synthetic steps. Never seen them or used them. Can think of all sorts of problems they could have, but have an open mind until I know more about them.
There is much to know about the proper way to drill and tap and protect the mast from corrosion. That's a whole different question. Have fun with your project, it is great you are taking it on.