Vince, I really like how you're asking all of the right questions for me! We must be at the same stage of the game.
I've given thought to this as well. Input I've had said that a glass top would not allow for beneficial gas/oxygen exchange. However, I don't want any fishies to take the leap. So it was suggested to me to have a canopy instead of a pendant. But in case I go MH, then I'd prefer a pendant due to the heat.
I'm still working on what lighting I want, etc., so I'm not to this point yet. But I do want my lights to work in some kind of set-up with a top (canopy, glass, etc.) I had at one point asked about a screen (top, yet gaseous exchange possible) but the replies indicated that a screen would screen out too much light.
I am now thinking about getting some plexiglass with holes in it (like the kind used for tank dividers)to allow for clear tank coverage as well as gaseous exchange.
Any more experienced out there have any thoughts on this?