Not tried a metal one but if I would it would be a stainless steel one. If you buy a metal make sure it isn't just chrome coated because that will just begin to flake after a few weeks.
Personally now if I was to buy a stainless steel one I would have it custom made. I have been in chastity since November and after trying a few cages the right fitting is very important. At least 1/2 inch shorter than your limp size. Believe me tighter means comfort.
If you search the net you will find companies that make these stainless steel to size.
As for plastic I am currently locked in the CB3000. After trying the CB6000 and CB Curve this is the perfect fit for me and I find this very comfortable indeed.
If you have room for growth you will find the cage will become very uncomfortable. Sometimes especially with my 5am wood I feel like the hulk trying to break out but so far have had no issue with splitting at the seams and believe me I consider myself to have a strong erection.
The CB 3000 and 6000 also tuck in nicely and is only really visible as a bulge in tight trousers. I wear throughout the day even at work and it remains well hidden. I also own the mystim cock cage which also is a great fit and remains as hidden as the CB's. Although I know when I'm in for some electro teasing because my key holder (oh) will tell me to put that one on instead of my usually CB 3000.
It is trial and error at first so maybe try a few of the cheaper options if only for sizing but if you are to be full time chastity the CB's are great but I am looking into a custom Stainless Steel one just for ease of cleaning whilst having a shower.
At the moment my key holder has to unlock me every other day for a thorough supervised clean because the CB's almost fully cover whereas the steel allow easier cleaning. My key holder also wants to be able to leave me locked whilst I shower so she doesn't need to supervise and a steel cage would make this possible.
We have only ever had steel cages which she likes the look of and I'm happy with the feel.
I started with a fairly long cage ( http://www.lovehoney.co.uk/product.cfm?p=28547 ) but as Stuburns says, too much room allows for more excitement. I often use the larger cage as part of foreplay so OH can get me excited and tease me through the bars and then unlook it for full sex.
One thing I would say about the metal cages, I would not recommend any that have a hinged ring to go behind the balls, I find the hinge at the base of the ring can pinch the skin (there are often rubber sleeves provided to slide over the hinge to help) and also skin can get pinched at the top where the lock goes.
I far prefer a solid ring and I think that is all LH sells
in the last couple of months, I find that it works quite well, and although if I really wanted to I could get it off even whilst locked, i do not see that as a bad thing as emergencies do sometimes arise. I find that it restrict erections well but is not noticeable under clothes. I do not wear it for extended periods, it is normally used during the day if i am off and my wife is at work to make sure i keep my hands to myself.
as for long term wear everything i have read suggests a slow build up to this, i certainly notice i am wearing it and would not want to be in it for more than a few days. My own advice would be start with a well reviewed item at a reasonable price and see how you find it, you might not like it and then look to have a custom device made if you are after something for long term.
I've been looking at the steel cock cages too, problem is the one I've seen on the net LH doesn't do!! The one I've set my heart on has a steel ball butt plug built in to it and a pee-through flexable urethral sound that screws into the end of the cage when wearing it! Just my kind of fun!!!