Glass tops aren't as evil as some people make them out to be... but you've got salt creep and mineral deposits that need to be cleaned regularly, and it does reduce the gas exchange. If you've got a skimmer, it won't matter... plenty of gas exchange in a skimmer. Glass isn't my choice, but it's not a bad way to go.
Screen tops are great.. keep the fish in, don't need more than a freshwater rinse every once in a while, allow for full gas exchange... but they look like crap. There are a couple of vendors selling some real nice looking screen tops, (artfully acrylic, for one), but they're EXPENSIVE. The DIY kits, I just don't like the look. You're running a canopy, or even a rimmed tank, where the screen top is either covered, or at least sitting in a molded in track... looks like it belongs, fine. Rimless tank? Just looks bad.
My tank is wide, fat, and low. Thick beveled and polished glass edges, rimless... I don't want a top. Any top. Certainly not an ugly one
Open top means researching the fishes you select carefully. Wrasses are a no no... as are most long, thin fishes. Dartfish, dottybacks, basslets... all are a fairly major risk. Even if you do carefully select your fishes, you can still have one jump. I lost an Ocellaris clown... not a fish commonly known for jumping. It can happen with ANY fish. A very peaceful environment, with as little aggression between fishes as you can manage, helps. Oh, I loose a snail once in a while, too... ends up on the outside glass, all dried up.
IMHO, if your tank has the 'open' look, an open top is worth it... It's a hobby. If you're not going to be happy with the look of your tank, why bother?
