Call Hans and pick his brain. Here was my experience:
18 vs 20 - I thought it would be best to have the extra ammo. He generally makes them in 18" b/c they swing faster. In a 590 18", using a speedfeed stock, side saddle and a + 1 tube - total capacity is 17 rounds. The extra 2" + the extra ammo out at the end of the muzzle does make it slower on the swing - no question.
870 vs 590: I think I asked him more questions about this than anything else. I mean an 870 IS the industry standard, right? The 870 through Hans is also more $$. I wanted to know why and which was "better". He said "do you like ford or chevy?". He did state that the reason why the 870's were more money was because he had to do more work to them.
Secondly, he said that although it would most likely never be an issue - 870's unlike 590s silver sodder many parts together. 590s screw them together. Therefore, *in the event* that a part broke on an 870, it would need to be sent back to Remington because that part group would need to be silver soddered back together. Mossy - screw off broken part, order, screw on new part. Now -how often does either break -never.
Other considerations for me on 590 vs 870 - value:
ghost ring sights: 590 - standard, 870 -extra $ (Ghost ring sights are a must have)
parkerized: 590 - standard, 870 - extra $
Heavy Barrel: 590 - standard, 870 - N/A
One last thing to think about: do you prefer low ready or high ready position? I like high - faster / more natural shouldering motion. If you like a pistol gripped shoulder stock (necessitates low ready position) you need to get an 870. Why? The safety on the Mossy is on top of the receiver - thus you would have to remove your trigger hand from the pistol grip to snick the safety off - way slower in 3 gun (or crunch time).
This was my experience in purchasing a Vang weapon to meet my needs as a 3 gunner. Your needs and experience may differ. YMMV.
Call Hans.
ps - I want to make it clear - Hans did not show a preference toward an 870 or a 590. I bought a 590 b/c it was cheaper when all was said and done. The ergonomics sucked - first thing I did when I got it was hack the stock 1.5" (I'm 6'2" btw). I think either way (870 / 590) it really doesn't matter - except for the low ready vs high ready / safety / pistol gripped shoulder stock.
good luck