I have a GLA dual stage co2 reg for paintball tanks that I'm using. Honestly this sounds dumb, but it's a pain to go to a decent paintball place to get it filled properly. I wish I just got a normal tank.
Get a good dual stage reg that uses a normal tank that you can get filled at a welder shop or a place that sells gas. Avoid cheap ebay junk or cheap single stage.
From my experience co2 is able to diffuse better in deeper tanks as it has more height to travel and absorb, your dealing with a 20 long, you'll want a darn good diffuser or reactor if you use paintball tanks.
Just remember, co2 tanks are about 850psi depending on the temperature, once they run out of liquid it drops the psi over a week or two, which then screws with the working pressure a bit and your bubble count. Even the expensive regs have this issue, while neglectable it exists. Some cheap regs also have end of tank dumps, but I'm not quite sure I firmly understand that.
What's your budget?