Im using a Dive Rite Plates and harnesses and a Hog 32 lb wing. Both are exceptionally well made and all 3 brands you have mentioned make excellent products. The real deciding factors for me where availability, Service, and Cost.
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You don't specify where you do your diving so I'm going to guess locally. I started off with a SS DSS BP with weight plates that I got off ebay, but since cold water diving wasn't for me, I sold it all. If you're a travelling diver and want compact and lightweight (and quality, of course), I'd recommend an OxyCheq rig. I have an 18# wing paired with their ultralight BP and find it absolutely perfect!
Regardless of what brand you decide on, be sure to check Craigslist. I recently bought a next-to-new Suunto SK7 in a DSS mount for $40.
LOL! I've had sketchy customer service from DSS, prior to even buying anything, so I'd only turn to that brand as a last resort.It didn't look like OxyCheq was on your radar from your thread title. I dive Oxy wings an 18lb for travel and a 30lb for local, tiny wings and very well made I love them. They have a reputation for sketchy customer service but fortunately I haven't had to personally test it.
Cost will depend on brand, I know for example AGIR BROKK make a 6mm plate weighing in at just over 5kgs, it's expensive as most AB products are.
As for "effectiveness" they will both do the something as both will put the weight over your lungs. A 3mm with weight plates will of course be more versatile in that you can vary weighting depending on trim requirements, exposure wear, tanks etc. obviously with a 6mm plate you are stuck with the fixed weight if the plate. Personally I'd go for versatility.