I just ordered a new 6 qt stockpot (that I use for soups and stews) and it came with a metal lid. All the stockpots I owned before had glass lids with a small vent hole to release some steam.
An obvious advantage of glass lids is that you can see the food being cooked and monitor the level of boiling. Are there any advantages of metal lids? E.g. do they keep more heat inside if you keep it on vs letting heat out?
I like my metal lid stockpot because it can go in the oven if I need it to. Glass lids can often go in the oven, but are sometimes limited to temps of 300-350. Many are 400, but you'd want to check.
As for heat, metal is a better heat conductor than glass, per unit of thickness, but given that most metal lids are thinner than glass lids, I'm not sure what the overall difference is. Probably not much, since you're providing a much better heat source from below.
You do get to see more with glass lids, helping deal with potential boil-overs. But glass lids are harder to clean - stuff gets stuck in between the metal edge and the glass lid itself and that can be tough to get out.
Lastly, I find that metal lids are in general flatter and easier to store than glass lids, though for all lids I recommend getting some stick-on coat hooks and doing this:
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They don't break if you drop it on the floor. That about covers the advantages of a metal lid here.
The "heat kept in" is largely a matter of the steam condensing on the lid, and is pretty much exactly the same for metal or glass lids, or a sheet of tinfoil over the pot. If they are on, it is, if they are off, it's not.
I like glass lids because you can see what's going on without having to "check" all the time. I like metal lids because they are more durable. Even the metal ring around glass lids seem to be more susceptible to bending and warping than a full metal lids.
