What are the benefits of wearing one versus the other?

I know personally that gas masks when under any duress are absolutely uncomfortable (however much preferable over getting a good dousing with CS gas.)  Physical exertion is also made even more taxing because wearing a gas mask causes one to feel as if sucking air down through a tiny straw. 

A respirator would seem the more likely choice for a shadowrunner-on-the-go.  But then again, there are no mechanics really to reflect that, so why a respirator at all if you can have complete immunity with a gas mask?

If you're talking about the gas mask on p.327, it doesn't give you complete immunity, just a +2 to your resistance.  A respirator can be much better with a higher rating, since it adds its ratings to resistance.  A hazmat suit is what you need to be immune to toxins, and that has the obvious disadvantage of being a big suit that you can't let get punctured.

Interestingly enough, p.244 contradicts the rule on 327, saying a gas mask makes you immune to inhalation toxins...  And p.327 is corrected in the errata to say immunity    So you were right, but I'll leave the above in case anyone else gets confused.  I'd say the big difference is that lots of people wear respirators around all the time in SR4 because the cities are hazardous to health.  Especially on smoggy days, respirators are a common sight.  Not so gas masks, so you couldn't really have one on all the time 'just in case' without attracting attention.  I might also smack you with a perception penalty because of your severely restricted peripheral vision if I felt like it :p

Gas masks, however, can be built with built-in eyemods, so shove that perception penalty back where ya pulled it from!

Seriously, though, you familiar with the current industrial and cyberpunk culture? For someone who's so pro 'cyberware is normal' and 'people will be walking around naked with glowing robotic arms in their stomach' you sure are behind on the trends!

Seriously, though, a Gas-Mask with eye mods is just frakken' cool. Especially with the right accessories, like a skin-tight body suit, 6 inch heels, and fiber optic hair.

Wave of the future baby.

I'm a pretty big fan of the internal air tank, personally. It's one of those things that doesn't look very munchy on paper, but it's saved my butt on numerous occasions.

I would definately agree internal airtank is nice to have and one of those things I always pencil in on cyber characters without much though.