Albert - I use Vallejo almost exclusively... and love them! They airbrush like a dream, but best of all I can brush them too, as in landing gear, wheels etc...
they are called the "enamels of acrylics" and I have to agree, they hand brush smooth without brush marks or the such (especially if you use their thinner), being they come in a eye dropper type bottle you can meter out the paint AND thinner without worry of ratios or wasting them. 3:1 ratio works perfect for airbrushing, right out of the bottle for brushing.
They do have two main variants in their paints.. model air and model color. I like the model air because I can brush it without thinner and spray it as well...
the only problem is that they aren't named standardly as in the RLM and RAL colors have different names... BUT, if you are interested in trying them (or anyone else for that matter) I have a conversion chart matching up to standard RLM, RAL and FS paint names I would be happy to forward to you...
If you have any questions about the Vallejo line let me know, Darson (member name here) is also a recent convert and probably be more than happy to answer any questions you have as well...
I can't comment on the Humbrol, I have a few tins but haven't used them yet... I have heard good things about them too, but I was sold on my first bottle of Vallejo and won't go back! I am slowly replacing the entire inventory with it...