Both cages I have are hard plastic. I'm looking for other people's opinions on the differences between plastic vs silicon vs metal, etc
For long term wearing metal is the only way to go. Tubes keep everything inside and are very difficult to keep clean. Most plastic cages are tubes for strength.
My sample size on plastic devices is a CB-Whatever and it sucked (frequently removal necessary for cleaning and it split causing me some physical pain), but if there are viable open devices (e.g. JosieLynn Jewell's Cheery Keeper though I can't attest to it), they could be ok. My sample size on silicone devices is a Bon4 Silicone which had a number of problems unrelated to the material. It had the same need for periodic removal for cleaning but also got grimy just due to the material. Metal is amazing assuming: 1) the welds aren't shit 2) you don't have to go through a metal detector 3) the device isn't freakishly heavy. (the latter criticism I've only really seen directed at Lori's custom devices) 4) you don't have a metal allergy
There's still very much a metal is superior mindset, and it's warranted as historically all the plastic devices were similar in design (i.e. cb6k, HT, etc). Stainless steel has a great history of being a great material for a device. Technology and Kink have really advanced now, and Nylon can do things in shapes it couldn't do before If you want something light in weight, plastic/Nylon is really a winner - it doesn't pull things down nearly as much with a full ring and cage coming in under 30g. I think the one metal base ring I have weighs more than that without the cage. It's metal-detector friendly, even with the magic locker, but if you're really worried about it, there are plastic locks too. Theoretically Nylon allergies exist, but I don't have any stories to confirm it Either way - If the device is open, you're going to be happier with longer wear times, and cleanliness. Dabble in a couple different cheap devices - you may not want to wear for more than a play session, in which case, long-term wear isn't a consideration, and get whatever design tickles your keyholders fancy.
The selection of a chastity device is highly personal but at the end of the day the only thing that matters is that it's comfortable and fits your lifestyle. With the availability of cheap knock-offs it's easy to try several devices and not spend a lot of money. There are many devices available for under US$20. As JosieLynn pointed out, traditionally plastic devices like the CB series and Holy Trainer have been full tube devices, which makes hygiene more of an issue. The same is true of a full tube metal device. Obviously there isn't much air being circulated and no matter what you do, there will almost always be a drop of urine left. My experience has been that the tube needs daily removal and cleaning. While I've never worn one the silicone Bon4 and Birdkeep have always looked a bit bulky to me. With the advent of 3D printing, there more choices in "open" tube plastic devices like JosieLynn's excellent Cherry Keeper, Evotion's Orion, and Custom Chastity's Saint. Plastic is generally much lighter than steel as well. For some, myself included, there is a seriousness, perhaps finality, associated with a metal device. Good luck in your search!
I've had both metal and plastic devices and currently prefer to wear the plastic holy trainer. It fits better for my lifestyle since it is lighter weight which makes it more comfortable for running. I also like the fact that it is a more closed device which completely prevents me from touching any part of my penis as opposed to my jailbird where I can fit a finger in between the bars. Like a few people have mentioned the holy trainer as is does not allow for very long term wear for most people because of how enclosed. To fix this I added 10 additional vent holes to it just by using a drill and widened the opening slightly with a chisel and sandpaper. I'm currently on 10 days locked without any signs of discomfort and without any urine being trapped in the cage.