Sizing plastic ring from MM

Which exactly are you trying to size? The interior diameter (ring on your penis) or the base ring (ring below the scrotum)?

Any erection in a sizing ring is going to be uncomfortable at best and painful at worst. In a properly fitting device, you're not going to have frequent erections, so that's less of an issue.

If this is going to be literally your first device, I would suggest trying out a cheap chinese device you can get on a site like Amazon. Ideally a smaller one with 2-3 different base rings. The MM sizing rings suggested that I could wear a 1 5/8" inch base and when I tried, nocturnal erections were excruciating while perfectly fine in a larger base ring.

 

Yes, the measuring rings are to determine your flaccid size, whether you’re sizing the base ring or the diameter of the cage.

I agree that if you’ve no experience with chastity you should probably start with one of the cheaper models. Mature Metal will absolutely help you get the correct measurements, but if you’re doing this strictly on someone else’s recommendation you may want to start with less of a monetary commitment.

 

The metal base ring is going to be a little bigger but not significantly.

You're not going to be able to pull off the device. You will be able to pull out the back, but the size of the base ring doesn't really change that. You need a full belt, a piercing, or a device with an anti-pullout ring to prevent that.

Erections are going to hurt when you're wearing a chastity device. That's a fact of life. A properly fitting device will

Expensive devices are more comfortable (better fit, better welds, fewer issues with the lock, etc.), not more secure unless they have something to secure the device to a piercing or an anti-pullout ring.

The ability to stay clean while wearing a device depends on the design of the device not how much you paid for it. There are plenty of open design style knockoffs.

 

I started with a CB-6000S clone and a Holy Trainer clone. The CB broke in a very short time and the Holy Trainer, while reasonably comfortable, irritated me in a couple of spots. Then I bought a cheap metal one and it's been so much more comfortable. The one thing I really liked about the Holy Trainer was how the tube fit my penis, most of the time I couldn't even tell my penis was in a tube. The shape of my metal tube is different because of the way it's constructed and I can definitely feel my penis moving around a lot more. It's not irritating or uncomfortable, just different.

I'm going to suggest skipping the plastic devices and going right to metal. Just pick a design that you like or think will work for you. The more open the tube, the easier it is to keep things clean but it gives you (and your key holder) more access to your penis. The base rings can be purchased separately from the device (definitely on the Chinese sites, less sure about Amazon), so for a relatively modest investment one can try different size base rings without having to buy several tubes.