HI Dave,
In my opinion, you don't want to apply either of these finishes if you are having to choose which one is the worse for your health. Only apply these finishes if you can do so while suffering absolutely no harm to your health. If you are protected, then it doesn't matter how many times you are going into the spray booth. In other words, you ONLY want to use these finishes if you can apply them safely. Both are quite toxic !!!
BUT I do know what you are getting at here. I think the catalyzed urethane might be worse - with the isocyantes in the hardeners potentially life threatening for some individuals who might be sensitive to it. A lot of folks spray this stuff and throw caution to the wind -- and they are fine -- and some get cancer or get rushed to the ER. I'm not willing to take ANY risk !!! I've got a sufficient spray booth that captures all the overspray. I'm wearing a Tyvek spray suit, latex gloves, and a full face 3M respirator with fresh cartridges. Nothing gets on me while I spray. My shop smells bad for a day but when I enter it -- I'm decked out in the PPE.
I think the Urethane might be a bit easier to apply just because you don't have to shoot as many coats. But then again, there's a bit more sanding involved because you must have mechanical adhesion rather than chemical adhesion between coats. But I've found the sanding fairly painless. I also think the urethane is a bit better or the environment -- the chemicals catalyze and therefore, lose their toxicity. You do have the solvent carrier but urethane is a fairly high solids paint. However, with the amount of finish we apply -- environmental concern isn't that high on the list.
I think the main reason lacquer can be easier to apply is that you never have to worry about witness lines - it's easy to spray and relatively easy to keep thin. It's super easy to level and buff. Urethane is a bit more work in some regards but also easier in others.
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One consideration is that VOC's are the things that linger in your shop and nitro has got more of those. Bar none, the worst thing about the urethane for me is the MEK smell -- it lingers but is mostly gone by the next day. If it helps any, I believe urethane is easier for me to spray safely. I don't believe I could apply lacquer safely in my current shop setting.
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