I have 3 dogs, all of which are crated when I'm not home. I've tried the wire type crates but overall just don't care for them for my crew. Instead, I use the hard plastic, especially since they seem to be more handy (and safe, IMO) when travelling.
It seems like a lot of people prefer the wire ones as opposed to my choice but overall plastic just works beter here.
Wire crates are better for ventilation and fold flat for easier storage and transport. They are also easier to clean because you can just slide that tray out and scrub it. The plastic crates are a lot sturdier. Unless you have a little purse-dog, they are a must for airline travel. If you live in a tornado prone area it's a good idea to have plactic crates for all your animals in your tornado shelter. If your house is blown away or collapses, that crate will protect them from being hit by debris or from panicing and running off. A wire crate will bend and jam the door, but a plastic crate will dent or crack and can be popped back out to free your pet. If all else fails, it can be unbolted and taken appart to free the door or get them out.
I have two crates. When traveling in the car, I use a plastic crate. My dog is small, so it's easy to carry in and out with her in it. I like this option for travel because I take my dog to family get togethers. When we eat, I can lock her in her crate so she isn't bugging everyone.
She still sleeps in a crate. For this, I have a wire crate. It has enough room for her bed and her blanket. She can sit up and move around easily. She gets enough ventilation, even though she prefers sleeping under the blankets. The only problem is it's heavy. At least, it's too heavy for me to move around.
We are currently crate-less because the crate I bought for Peanuts was too small for her. I tried introducing Chooey to the crate but she would only follow Peanuts wherever she sleeps. And the plastic crate is a bit hot given that we currently have a hot climate.
I prefer wire crates because they are definitely cooler. But for transporting my dogs whenever we travel, plastic crates are the best. And they're only the ones allowed when flying by plane.
I have one of each; a wired cage and a hard plastic cage. The wired cage is cool because it collapses down to nothing when we go on vacation. I use the hard plastic cage at home just because it seems more comfortable for my dog. I also think it is a little bit bigger as well.
I only use my lab's hard plastic crate when he is traveling. Right now the rate is occupied by my rabbit since it is to hot outside for her in her hutch. She likes it very much. I feel like plastic crates are a bit more durable and they are also weather proof if the crate is outside. They are usually bigger than wire crates, to.