I have used glass tops on my tanks for a while and I decided here a few months ago to switch to Egg Crate because I wanted to try something. The main reason was it was hard to keep the glass clean and it was simply a pain in the neck to was it and scrape off the residue.
Since I have switched to egg crate I have seen a noticeable difference in plant growth. My wisteria is growing close to the substrate instead of growing up into a massive mess. Also, my Bacopa is growing much thicker and grows fuller instead of racing to the top with the bottom leaves falling off.
I do have more problems with algae but this would also indicate that a lot more light is getting to the tnak than when I used the glass tops.
I think for light penetraiton, the egg crate has made a huge difference. I do have a little more evaporation but I change 50% or more weekly so it really doesn't hurt anything anyway.
Anyone have similar experience with egg crate or seen any differences between different types of tank covers?