I am currently based in angmoh country. My dive buddies here are keen contributors to scubaboard. One observation I made, comparing the dives in ang moh country and in SEA is that, they use lots of weight.
I remember my first time diving here, and they asked me how many weights I need on my belt. I shrugged and said none. I was diving with a 3.2.2 mares wetsuit, with a 2mm lavacore hood, and a halcyon aluminium backplate. I remember the instructor's face. His jaw nearly dropped to the floor. He kept reminding me that we are doing sea diving, and not in fresh water so I definitely need 10lbs of weights! I laughed it off and ignored him and he wasn't very pleased. He brought an extra weightbelt with 10lbs of weight on him just in case he needs to pass it to me because he is certain I will not be able to descend. I remember chuckling the entire dive because he was 10lbs overweight and his buoyancy was all over the place (due to additional weight).
Asian people generally need less weights. All the angmoh divers here, no matter how fit they are, with a 5mm wetsuit & jacket bcd will need at least 12lbs. And they all look at me in awe coz I never had to lug around a weightbelt for shore entries. Therefore, I believe the people in scubaboard a little "kua zhang".
On the other hand, I can only tell you to try it out (both plastic and SS) and adjust accordingly. There is no hard and fast rule when it comes to perfect weighting and trim. It may involve a lot of trial and error. Even if you got the perfect weighting checked, there is still an issue with perfect trim (eg: where to put the weights).
If you are a medium build guy, I would suggest you to get an aluminium plate (not too heavy, not too light) and start playing around from there. I would also suggest you not to put your weights on the cam band (it may work for you - but i hate it because when roll around in it, I do not feel that it's very stable). If I need to use weights (when I'm using a 7mm I use 4lbs). I have 2 lbs on each side of my lower back, with the tank in between it. But this came from a lot of trial and error, and I know some people don't feel comfortable with the weights there.
Buy what you think is right, but do not expect it to be perfect the first time you go diving in it.