The mirror is going to be the equivalent of window glass. There is no advantage whatsoever in making the top a mirror as opposed to standard window glass (other than appearance). Don’t believe me? Buy a cheap mirror and smash it.
The mirror does have a beveled edge. That’s safer than the sharp edges on standard window glass. But you’re correct that it’s the same thin and fragile glass. Some of the comments indicated people do use it as a table top centerpiece.
Ace, get the glass made for flat surfaces, I can’t remember the details, but I asked once at our local glass and window shop what the difference was. Thickness was the most obvious but the shelving/tabletop glass had a manufacturing difference also, just can’t remember what. A half inch thick piece of glass for a table is the table top itself.
OH, how about this, is there a local cut and sew shop for marine canvas (boat toppers seats etc) near you? I bet you could get a piece of isinglass for pretty close to nothing from them if you were willing to go that route.
Looks like you have already come up with a solution, but I can chime in as a “no” to regular glass. I acquired a 5-shelf unit made from curly-que type heavy wire from a relative who brought it back from Turkey in the 1960s. It had heavy tempered glass on each shelf originally, and after 40+ years of her moving around, 3 of the 5 pieces of glass have been lost and broken.
The regular glass I had cut to replace them has broken, twice. I should have sprung for the extra $$ for tempered glass to begin with.
If you want glass go to the glass shop and get the proper glass. Plastic will get scratched easily. A simple coating with polyurethane will protect it, be fairly resistant to marring and waterproof, but refinishing will be needed eventually, still at low cost with minimal effort. Epoxy coatings are tougher and longer lasting but not as easy to get right if you do it yourself. A couple of coats of Tung Oil and some buffing will give it a decent surface that is not hard to refinish as well. If you really like the table the glass top will be worth it in the end, but if you aren’t sure if you like it that much yet take an inexpensive temporary measure (not window glass) and get the proper glass down the road when it makes sense to you. You may need to put felt pads down for any solid top you add, even if the top seems perfectly flat now it may not stay that way, although the weight of 1/2" glass will help keep it flat if it already is now.
You really should spend the money on a sized piece of glass from the glass shop. It will be “custom” because the glass will be tempered glass and tempered glass cannot be cut after being tempered. You really do want tempered glass on the table top because tempered glass is up to 7 times stronger than regular glass. The tempering makes the outside layer of the glass stronger and holds the interior of the glass under tension pressure. There is more science behind it but that is a good way to think about tempered glass.
When or if tempered breaks you get a bunch of corn kernel sized pieces, when regular glass breaks you get large, daggers of danger. What do you really want in your house?
I’m not sure tempered glass is the right thing here. Just placing any hard object down on it in just the right way can cause tempered glass to shatter. 1/2" glass is extremely strong already. I would say follow the recommendations of the glass shop in this regard though.
ETA: If the top were made only of glass then tempered would be the right choice.