how much time you have to spend on a custom cage. Custom cages with sliding or hinged doors on the front are probably much better than glass tanks with screen tops. for one thing, the screen tops make them horribly inefficient for retaining heat, and it basically makes a big jerky maker. What I mean by that is that you just keep heating up the animal, but the heat is constantly rising up and out, and it's taking with it moisture from your lizard(s). That's why in my uromastyx tank I have almost the entire screen top covered with tin foil (I'd use plexi, but I don't have any lying around). The only parts not covered are holes just big enough for the lights, and one 3"x3" hole for ventilation. With this it makes it much easier to maintain the temperatures necessary for a uro (a beardie is not quite as demanding as far as temps go though), plus the soil I use in her tank isn't constantly drying out anymore, which is good. She's also acted a bit healthier since I've made the change.
So anyways, I've used glass tanks for all of my reptiles, but now I'm building some nice custom enclosures for them, and I think they will work out much better. All I'm putting in for ventilation is two 3" round holes. I'm also planning on using a thin layer of soil for my beardie, and a much thicker layer (up to 10" or so....thick enough to dig a small burrow) for my uromastyx. I think the setup will work out well, but I'm obviously not certain yet. I'm trying to just give them the basic resources of what they would have in the wild I guess. I'll let everyone know how it works out once I get it all up and going.
I think you may have gotten my point, though I went off on a tangent there. Basically, aquariums with screen tops can be used, but they really aren't as good. If you don't have time to build a custom cage within the next month or two though, I'd say go get a 40 breeder and then plan on building something bigger at a later date. That's basically what I did. I wish I had just gone with a big custom cage in the beginning though.