We have been doing this, off and on, for about 10 years and have tried multiple types of cages. IMO, you are doing the right thing to experiment with different types. It seems like trying to chase a moving target when trying to find the right sizing because it changes with not only the seasons and ambient temperature but with your KBT's getting more used to wearing devices. It's sorta like getting your fingers acclimated to the strings of a guitar, I suppose.
The plastic tube actually felt better than the metal to me but was very problematic where cleanliness was concern. In our early going, my Keyholder/spouse unlocked it, pulled it off, then literally ran the other way because of the smell. That, in itself almost derailed our chastity adventure just as it was just beginning. On top of that, you must keep the tube lubricated on the inside or it will not be comfortable to wear. However, the plastic tube kept me aroused more than any device I've tried and worn since. When I attempted to erect, it felt like her hand squeezing my shaft which aroused me even more. I was constantly dripping inside that device. Bottom line, it is good for short term lockups as long as you clean and lubricate regularly, and do a thorough cleaning before playtime.
If you are looking to incorporate male chastity into your ongoing, everyday lifestyle, then the MM Jailbird or a similar custom device is the way to go. (Notice I did't use the word permanent, don't wish to scare KBT

). We went to the custom device too early and, as a result, ended up purchasing several before getting the fit right. The JB doesn't have the "sexy feel" of the tube device but my feelings are not what this is about. My spouse started out doing this to please me, but she had to find something in it that turned her on to continue. Much to my chagrin, her turn on turned out to be the denial aspect. I have always masturbated a lot she always said our sex always seemed to be about my climax. Now, it really turns her on to deny me while at the same time, have me give her many orgasms. She tends to favor long term orgasm denial. It really turns her on to ask me, " Why do I have you locked up?" and I answer, "Because I masturbate too much". After she has denied me for several months at a time, and I resort to begging, she will simply say, "I'm the only one that cums in this house!". Just say it turns her on, and, me too.
Anyway, I missed welcoming you to the forum, Ms Kitten. I love seeing new Keyholders join because we need their perspectives and opinions badly on this board. Good luck on your and KBT's journey.