Errr, let's see...
The rap4 tanks are not exactly anti-siphon in fact they don't use a pin valve, instead the knob on the far end is an on/off that's piped through the center.
They dont have a standard burst disk, the orings in the on/off perform this function more expensively. Because they don't have a burst disk showing many places won't fill them, that's why people piggyback them from, in my case, a 20ozer. I get about three fills that produce 50 or so shots from my zeus and then it drops off to 20 unless I top off the 20ozer. It is steel and built like a brick outhouse.
The spare air type gameface cylinders(crosman makes one too) are steel and weigh as much as the rap4 tank.
I like the catalina 3.5 the best on my pistol, even though I had to go with a higher scope ring it balances as well. It gives a ton of shots.
Oddly I found that with these tanks I actually ran out of air on the field once in a while. Whereas I can carry as many 12gs as I could possibly use, I could only carry a few of each of these, plus at $50 a pop for the Rap4 30g and $25 average on ebay for the 3.5 I only bought a few of each. I planned to keep buying more of whichever I liked best, but I have to admit 12grams are way better for my needs(pistols) than either of these so I stick with 12gs now and my tanks wait for the next project.
We play Pistol and we encourage the Few despite the discouragement of the Many.