Your Thoughts Please

One is siphon-feed and the other is gravity-feed so it is an apples to oranges comparison in my opinion overall. I would personally go with the Omni 4000 as you probably don't need the volume of paint that a siphon-feed can hold in a jar.
The Omni will also spray better at lower pressures and is a little easier to clean, although not much.
Hope this helps.

I just got an Omni 4000 a little while ago, haven't used it too much, but I already absolutely love it. I was a modelrail—wish I had the time, now. Some day… anyway, I think the Omni will serve you very well in HO or even N scale. I can't believe how small I can get the lines! And it's a positive joy to clean.

There was a lot of discussion on this not too long ago. Search the forums on "airbrush".

Ideally WC, 1900-1940.  Obviously there will be some SOO, CNW and MILW.

weathering boxcars? Cool! What railroad do you model? I do the Western Pacific from the late 60's/early 70's myself.

as for the airbrushes, same as above, I've heard both are quite good.

 

I have an Omni 4000 and WOW I LOVE it. if you do low PSI and flip the tip on the A/B around you should get a fine line, And as for the other A/B sorry to say I do not know anything on that one

Good Luck

Bud

Either would be excellent. I've heard only good things about both brushes, with maybe "more" good things about the Omni 4000.